Difference between revisions of "Electrical systems"
Latest revision as of 15:21, 16 December 2013
This article is a travel topic
Electrical systems differ around the world - both in voltage and less critically, frequency. The physical interface (plugs and sockets) are also different and often incompatible. However, travellers with electrical appliances can take a few steps to ensure that they can be safely used at their destination.
 Voltage and Frequency
Start by taking a look at the back of the device you want to use. If it says something like "100-240V, 50/60 Hz", it will work anywhere in the world with the right plugs. If you've got both covered, you can skip to the next section. If not, keep reading.
Dealing with electricity differences can be daunting, but it actually isn't too hard. There are only two main types of electric systems used around the world, with varying physical connections:
Occasionally, you will find 100-127 volts at 50 Hz, such as in Tokyo, Madagascar, and some Caribbean islands. On the other hand, there's 220-240 volts at 60 Hz, such as in South Korea, Peru, some states of Brazil and Guyana. A few other countries using 60 Hz are internally divided, with 100-127 volts in some locations, and 220-240 volts in others, such as in Brazil, the Philippines, and Saudi Arabia. Be extra careful each time you travel to a new destination within these countries, and ask about the voltage. Be aware of multiphase electrical systems (see below under Large Appliance Power).
If the voltage and frequency for your device is the same as where you are travelling, then you need to worry only about the physical plug. (The small difference between 110V and 120V is within the tolerances of most electrical devices. Likewise for 220V and 240V.)
If the voltage provided by the local supply is not within the range accepted by your device, then you will need a transformer or converter to convert the voltage. Most travel accessory sources offer them and come with several plug adapters to solve all but the most exotic needs.
 Plugs and adapters
A device that lets you insert a plug into a different socket is an adapter: these are small, cheap and safe. For example, between Britain and Germany, you need only an adapter. You stick your British plug in the adapter, which connects the rectangular phase/live and neutral prongs to the round German ones and puts the ground where the German outlet expects it. Then, you're good to go.
Unfortunately, there are many different plugs in the world. The three most widespread standards are the following:
If your device has one of these plugs and you can adapt it to the others, you have 90% of the world covered. (The main exceptions are South Africa, Australia, New Zealand, Argentina, and parts of China, which use a Type I plug with two slanted pins.) Adapters between Type A and Type C and from C toG are tiny and cheap; converting Type A into G or Type G into anything else, on the other hand, needs a bulkier model.
For hobbyists: if you can't find an adapter, and you're staying for a longer time, just buy a separate plug at your destination, remove the existing plug, and attach the new one. Unlike adapters, plugs are always available, and they're generally cheaper too. Caution: try this only if you know what you're doing! (Fire and/or electrocution are possible if you are inexperienced.)
As a last resort, a Type C plug can be forced into a Type G socket without any converter at all if you ignore what your mother told you and stick a pen or similar pointy object into the center (ground) hole, which fools the socket into thinking a ground pin has been inserted and opens up the other holes. (This is, in fact, exactly what cheap C-to-G adapters do.) Disable the power to the socket and try to use something non-conductive (a dry non-metallic object) to do this! This procedure will damage the socket and could be unlawful in some countries, so expect the owner to be displeased.
There's one more complication to consider: any two-pin socket is ungrounded, but all three-pin plugs are grounded. Trying to get grounding to work makes life more difficult, as any of sockets C, D, E, F, H, J, K or L will happily accept the ungrounded plug C but will not work with any grounded variant other than their own. Do not use an adapter to turn a three-pin into a two-pin: this will disable grounding, potentially leaving you vulnerable to electrocution and other electrical nastiness.
A last word of warning: many developing countries use multi-plug sockets that accept (say) both Type A and Type C. Don't assume the voltage is correct just because the plug fits, since a Thai Type A+C socket still carries 220V and may destroy American (110V) Type A devices.
 Transformer or Converter?
If you are using a 110V appliance at 220V-240V, you can also use a transformer but may be able to get away with a (cheaper) converter.
If your device is an electric appliance with a heating element or mechanical motor such as an iron or hair-dryer, then you can probably just use a converter, but make sure your transformer or converter is fully rated to deliver the amps and watts your device needs. If your device is electronic and uses electronic chips or circuits, such as a computer, printer, TV, microwave, VCR or even a battery charger, you will need a transformer.
Before you purchase what you believe is your solution, understand the discussion below.
These lighter-weight, less expensive devices can handle large wattage loads of up to 1600 watts but only step-down voltage, not raise it. They are suitable for those in 110-120V countries traveling to where the voltage is 220-240V. Converters are designed to operate for only an hour or two at a time, not continuously. As stated above, they cannot be used with electronic devices: devices that use chips or circuits, such as a computers, printers, VCRs ,or even battery chargers.
Nowadays, many electronic devices actually come with a converter which plugs into the power mains and converts the current to DC. However, if this won't accept a foreign voltage (check the plug) do not place a second converter behind it. You must use a heavier transformer instead. Fortunately, in the past few years, more and more devices come with a universal AC/DC converter already included, and the most you would need is a plug adapter.
 Frequency (Hz)
Frequency is generally not a problem--most travel items will work on either 50 or 60 Hz. If all the electrical appliance does is produce heat or light (except fluorescent lighting), then the frequency is unlikely to matter.
Frequency is most likely to affect clocks and devices with motors. They may run faster or slower than they should and may be damaged in the long run as a result. Again, though, some motorised devices may function correctly on either 50 or 60 Hz-- especially if they also operate on batteries. Just look on the label or plug.
However, you still may need to be careful if you have a sensitive or expensive device that converts AC (power from the wall) into DC (battery-like current)--especially if you also need to convert the voltage. A device will convert AC to DC either to save battery power by allowing you to plug into the mains or charge a battery in the device. The design of power supplies where AC is converted into DC does take frequency into account.
Even though 60 Hz converts a little more easily to DC than 50 Hz does, there's enough tolerance in most small appliances and electronic gadgets that you can ignore frequency. However, if you also need to change the voltage (because the voltage of your device is different from the mains power voltage), you cannot use a switching-type converter. You must use the heavier iron-core transformer. If in doubt, consult a reputable electrical goods dealer.
If your device won't operate with a different frequency (powerful motors and non-quartz clocks), there is really nothing you can do to change it. Unlike voltage, frequency cannot easily be converted. Foreign embassies may have to use huge generators to provide current compatible with equipment from home.
If you desperately need to have power at your home country's frequency, you might try using a 12V DC to AC converter intended for vehicle use. However, most of these (especially those commonly found in stores) output a "sawtooth" wave instead of a sine wave. (Check the manufacturer's website if you need a sine wave output. It may be special order.) Make sure the wattage of the converter is sufficient for whatever device you need to operate, and the 12V battery has enough amps for the job. For example, 12V times 15 amps gives 180 watts (or less after losses are included).
Japan is a special case. East Japan (eg Tokyo) uses 50 Hz and west Japan (eg Osaka) uses 60 Hz. Equipment made for the Japanese market may have a switch to select 50 Hz or 60 Hz.
 Unstable supply
In many developing countries, electrical supply is highly erratic and you need to take precautions to protect your equipment.
The main danger is power spikes, where the voltage supplied temporarily surges to dangerous levels, with potentially catastrophic consequences. In developed countries, the main source of spikes is lightning strikes, but, in developing countries, they're most often associated with power outages since when the power comes back on, it rarely does so smoothly. The cheapest method of protection is thus simply to disconnect electronic devices as soon as the power goes out and wait a few minutes after the power comes back on until plugging them back in.
Surge protectors are devices designed specially to protect against spikes and surges, and some are available in portable travel-sized versions. Some surge protectors can also be fitted to a telephone line to protect your phone or laptop modem. The most common variety use a metal oxide varistor (MOV), which shorts to ground if a given voltage is exceeded. These are easily destroyed by larger spikes, and better models will have a light indicating when the MOV has broken down, but you still need to keep an eye on them as the device will still continue to give power even if the protection is gone. There are also surge protectors with fuses, which are fail-safe (a blown fuse will stop power) and replaceable, but there is still a risk of a short, sharp spike which can pass through and damage your device before the fuse blows.
In some (mostly poor) regions, you may experience electricity voltage drops. Instead of 240V for example, you may get only 200V or even less (50% of the nominal supply voltage is not unknown). This happens especially if you're at "the end of the line" (far from the source or transformer) and is caused by the resistance of the electric lines themselves. Some appliances, such as light bulbs and heating equipment just keep working under a lower voltage, although a 20% voltage drop will cause a 36% power drop. Most electronic devices also keep working, but voltage drops are critical for fluorescent lamps, refrigerators, and air conditioners which may stop working altogether (usually without being damaged: when the voltage returns to normal, they will start working again).
Voltage drops can be solved with a special device called a voltage stabiliser or AVR (Automatic Voltage Regulator). A stabiliser will raise the voltage again to its normal level. The principle is the same as for switching converters, except that stabilisers will produce a stable output, even with an unstable input. Stabilisers come in different power ranges, but they're all large, bulky and not practical to carry around. Be aware that some appliances, such as refrigerators, briefly consume 2 or 3 times more power at start-up; the stabiliser should be able to provide this power. Voltage stabilisers can introduce surges if there is a power outage. The cheaper and most common relay type can also damage electronic equipment.
If you are buying new appliances, get in the habit of checking the voltage. A dual-voltage hair straightener will cost you no more than a single voltage one, and save considerable hassle when travelling.
 Laptop computers
Virtually all laptop computers (including those with internal power supplies) will handle well a range of 100 to 240 volts and a frequency of 50 to 60Hz. In other words, you won't need a converter/transformer; most power supplies have supported ranges printed directly on them so have a look. You will still need to check that you have the plug that matches the outlet for the country you are going to to see if you need to buy an adaptor.
Laptop computer power supplies are generally very good at accepting a poor or varying supply. Many manufacturers use the same type of supply, so getting spares is not too hard. The type used by HP/Compaq is very common. It is very easy and cheap to get a spare supply from sites such as Ebay. However, make sure it is a genuine manufacturer replacement and not a cheap copy. With a spare, you can take a risk with an unknown supply. Of course, do not take any risks if your laptop is one of the few with an internal supply.
If you are taking a laptop, you can use it to charge other items using a USB port on the laptop, even if they are normally not connected to it - this can save you a bundle of transformers in your luggage. Just make sure you have the correct USB cable - there are many different types.
Radios also tend to be interchangeable from country to country. The exact FM range being used in a few countries is different, so you may not be able to access all stations. In the US, only odd channels (88.1,88.3, 100.1 etc) are used. A radio intended exclusively for the US market will not work well in most other countries. Japan, in particular, has an FM band from 76 MHz to 90 MHz rather than the more common 87.5 MHz to 108 MHz. The countries of the former Soviet Union have also used a similar band. For the medium wave band, channel spacings (the difference between each valid frequency) can be 9kHz or 10kHz (for USA). Some digital radios will have a switch or setting to choose which channel spacing is used. Without this, they will not work correctly outside their intended market. Old-fashioned analogue dial tuners don't have this limitation.
If you need a new radio for international travel, consider one that includes the shortwave band (SW). This way, you can receive news and information from all over the world (BBC, CBC, Voice of America, Radio Australia, etc.) Shortwave is above the medium wave band (in frequency terms), but travels a lot further, especially after dark. In the past few years, the size and price have come down considerably for AM/SW/FM radios, and they are much easier to use. A handful of stations now require the single sideband (SSB) function (normally used by hobbyists for voice communication), but for most people it's not essential.
 Digital Radio
Digital radio is in use in some countries, but has generally not attracted large audiences. So radio listening remains a predominantly analogue world, unlike television. The most common systems are DAB (Europe), DAB+, DRM and HD Radio (US). For travellers, an analogue radio (especially those with digital displays) are the best choice.
 Mobile phones and digital cameras
Chargers for these may work with both 110V and 240V systems, though you may still need an adaptor plug or have to use the shaver socket. You may be able to get a second charger for the other voltage system, or even a dual voltage charger designed for both systems. However, your mobile phone handset may not be compatible with the country's network, or you may be limited to certain cellular providers. (See Telephone service for travel#Cell phones.)
 Equipment using standard batteries
Battery sizes and voltages tend to be standard from place to place, and equipment that uses off-the-shelf batteries tends to be interchangeable. It may be difficult to get good quality batteries in some countries, especially alkaline batteries which are needed by most electronic equipment. If a cheaper battery is used, make sure to remove it as soon as it is exhausted or if the equipment will not be used for a while due to the risk of leakage.
Dual voltage battery chargers for NiCad and NiMH generally cost no more than single voltage ones, but you need to look for this feature before you buy. If an existing single-voltage charger uses a 12 volt DC adapter, find a quality dual-voltage adapter (110V - 240V) at 12 Volts DC with its DC current rating (in milliAmps) equal or higher, and the same size plug on the charger end. (This is not possible if the charger plugs directly into the power mains without any cord.)
 Be cautious
 Large appliance power mains (Single, split phase or three phase supply)
In most countries, electrical power is distributed using a three phase system. This means that there are 3 different live/phase wires and optionally a single neutral wire. Domestic outlets are invariably single phase. A domestic outlet will receive just one of these phase wires and a neutral wire. Depending on the country, a mains supply into a residential building may be a single phase supply or three phase supply. Most larger buildings will receive a three phase supply.
There are good reasons why electricity networks use 3 phase supplies. They are also most appropriate for running large machines with motors (large air conditioners, industrial/commercial ovens and other power hungry appliances). The voltage between any of the three phases and neutral is the same as the domestic single phase out let voltage (110V, 230V, 240V etc). However, the voltage between two phase wires is typically 380-415V or 208V in a 110V single phase system.
In North America there is a further variation, the split phase system. In the split phase system, two phase wires are used, but the voltage between these wires is double the single phase voltage - 240V. These systems are used to run large appliances such as cookers. However, it is also not uncommon to use two of the phase wires from the three phase system to give 208V.
In some places (mostly industrial buildings) three phase outlets may be available (often coloured red). Adapter cables or plugs can be used which take one of the phase connections and neutral for standard single phase appliances. Regardless of how many phases are in use, the frequency (50Hz or 60Hz) of the supply remains unchanged.
In countries with a poor distribution network, it is not unknown for single phase sockets to be connected across 2 phases to boost the voltage. This is dangerous and can damage electronic equipment. As a general rule, do not attempt to connect your personal electrical items to a socket or wiring system that is a three phase system.
In many countries without fully developed electrical power distribution systems, the use of generators is common. Generator supplies can be very good; however, in many places they are bad and can cause damage to sensitive equipment connected. The voltage, frequency, and waveform shape (it should be a smooth sine-wave) can vary. In some places, people modify generators to run faster. This gives more voltage and power but increases the frequency too. The part of a generator that keeps it running at a constant speed is called the governor. If this is tampered with, the output voltage could rise sufficiently to cause damage. The best advice is not to connect valuable equipment to the supply, or at the very least disconnect it as soon as you're finished.
If you are unsure about the quality of the generator in use, there are a few simple rules. If it runs from petrol/gasoline, it is bad: anyone serious about using generator power uses a diesel oil powered system. A good quality generator will have a low engine speed. 1500RPM for 50Hz or 1800RPM for 60Hz. If the engine speed is 3000RPM or more, it is not a good machine.
Lamps and their light bulbs are very sensitive to voltage. If you shift between voltage systems, you will need to change the light bulbs to match the voltage, unless the lamp is designed to operate on both systems, say through a low voltage adaptor. If you buy a lamp abroad, you may need to have an electrician completely rewire a lamp when you get home to comply with your country's electrical safety standards. This may not be a problem for a one-off special item, but if you are going into the importing business it could be a showstopper.
Also watch out for the light bulb connection. In 100-127V systems this is often a screw connector while in 220-240V systems it is often a bayonet connector. These connectors also come in at least two different sizes. Be sure you can obtain light bulbs of the right voltage, size, and connector shape in the country you intend to use the lamp, and at a reasonable price, or the lamp may become little more than junk when the bulb fails.
Note that fluorescent and LED lighting contains electronics and must use a heavy iron-core transformer to convert voltage. Converters are not acceptable. Some fluorescent units might be sensitive to changes in frequency (50 or 60 Hz) if it's not the same as what is specified. This type of lighting has its own "flickering" frequency, which is suppose to be too rapid to notice. However, old and defective units often produce an annoying, visible flicker, and the wrong electrical frequency might have the same effect as well.
 Electric motors
The electric motors in things like refrigerators, vacuum cleaners, washing machines and other whiteware are often sensitive to frequency. Older hairdryers and electric shavers might be also. Even if you use a step-up or step-down transformer, the different supply frequencies mean that motors will run at the wrong speed and quickly burn out. The larger and more powerful the motor is, the more this is true. Don't, for example, bring a vacuum cleaner from the US to Europe (or vice versa). It's almost guaranteed to fail--even if you have a voltage converter.
 Electric shavers
Hotels often provide a special electrical outlet specifically for electric shavers. They allow any voltage shaver to be plugged into them and be used safely in front of the bathroom mirror. They may also accept your cellphone adaptor or similar low power battery charging unit. Many--but not all-- electric shavers sold today are dual voltage 50/60Hz and some will even recharge the battery at 12V DC (such as in an automobile). Check the label and instructions for compatibility.
Hairdryers are a particular risk; if you accidentally plug your 100-120V hairdryer into a 240V outlet. you may find it catching fire in your hands! Newer models should have a thermal switch, though. Allow 15-20 mins for it to cool down, then use a voltage converter (if the dryer is 50 Hz compatible). Similarly, a 220-240V hairdryer in a 120V outlet may run slowly and not heat up enough. Most good hotels and motels will be able to supply a hairdryer, and it may even be a room fitting. However, it may be worthwhile buying or borrowing a hairdryer suited for the electrical system of countries you'll be traveling in.
Many new hairdryers sold in 100-120V countries are dual voltage with settings for 100-120V and 220-240V. Even though it's motorized, it will work on either 50 or 60 Hz. Don't forget to lockout the high setting with a flat screwdriver or something similar. At 220-240V, the low setting becomes as powerful as the high setting was at home (with 'low' unavailable).
An electric clock of any sort is sensitive to voltage. If the voltage is doubled or halved, it will not function and may burn out. Furthermore, the electric frequency (50 or 60 Hz) is used in cheap clocks (such as many clock-radio style clocks) to keep the time. Thus, if a clock made for North America were used in Europe – even with a voltage adapter – it would lose 10 min/h! Obviously, not a great idea if you have a train to catch. On the other hand, if the clock has a quartz crystal, this is used for the timekeeping, and it operates independently of the line frequency. Inexpensive, battery-operated, digital LCD travel clocks (with a push button back light) are also available. These are recommended for destinations with frequent blackouts.
 Video equipment
Televisions, many radios, video and DVD players, as well as videotapes, are often specific to the broadcast system used in the country that they are sold in, usually associated with the frequency of the country's electric current. For example, North America is 60 Hz and its television is 30 frames per second, while Europe is 50 Hz and its television is 25 frames per second. The main three analogue television broadcast systems are PAL, the closest to a worldwide standard, NTSC, used mostly in the Americas and some East Asian countries (notably Japan, South Korea, and Taiwan) and SECAM, originally from France and adopted by much of Eastern Europe and the Middle East, but there are various incompatibilities even within these supposed standards. There is no difference between PAL and SECAM for unconverted DIGITAL video including DVDs. However, any analog output to a television set would be in the native format of the country of location. Brazil uses a hybrid PAL/NTSC standard called "PAL-M". In Brazil, DVDs and video tape are the same as NTSC (without region coding- see below), but all players and TV sets are useless outside the country unless they have a separate NTSC setting.
Before purchasing any video equipment, read the manual and warranty carefully. For TVs and VCRs, don't forget about cable television frequencies; they may not be the same, even if everything else is. Television sets often won't work correctly in another country from where they were sold, even if the voltage and video standard are the same. For example, a television set made for the USA will skip a few channels in Japan. Furthermore, many countries have or are in the process of switching to digital over-the-air broadcasting, (dates by country). Unless you have an internationally compatible device, you may find your expensive looking system is little more than worthless junk in another country because it won't work with your country's broadcast system. Your warranty is probably only valid in the country of purchase, and you may need to return the goods to the place you purchased them from.
The final problem with transporting TVs is that many European countries, notoriously the UK, require a license to watch any live TV (over-the-air, cable, satellite, and even live-streams on the internet). Fines can be hefty (in addition to being charged for the license).
DVD and Blu-Ray, infuriatingly, have completely artificial limitations introduced in the form of region coding, which attempts to limit the region where the discs can be used, as a technique to keep the various regions as separate markets. For example, a Region 1 player for North America will not play a Region 3 DVD for Hong Kong. The workarounds are to obtain either a regionless DVD player which ignores the code, purchase multi-region discs (Regions 1 and 3 in this case), or better yet, Region 0 discs, which can be played on any device. Blu-Ray discs cannot be played at all in a standard DVD player -- not even at a lower resolution. However, Blu-Ray discs played on a Blu-Ray player can be displayed on a standard def. television, provided you have the correct cables and connections. (HDMI cables are not compatible.)
Technically, there is no such thing as an NTSC or PAL DVD disc, as all color information is the same for both. When discs are labelled as such, what they're referring to is the picture size and frame rate (i.e. number of frames per second) that are used in most (but not all!) countries that have TV broadcasts on this same system. Many NTSC players cannot play PAL DVDs, unless that's a specific feature included (many Philips and JVC models include this). PAL DVD players are generally much better at playing NTSC, but it's not a certainty. If all else fails, a computer DVD-ROM can play any DVD movie, though there's a limit on how many times you can change the region code. Unlike analogue television sets, computer monitors can automatically handle both 25 (PAL and SECAM) and 30 (NTSC) frames per second, as well as various picture sizes. This also applies to LCD and plasma "flat panel" television sets, but don't expect their tuner to be compatible outside the country in which they were sold.
Video cameras can usually be charged with both electrical systems so you can record during travels and view it back home. Digital cameras and video cameras can usually output to both PAL, NTSC, and SECAM, so you can view your recording while travelling. Bring an RCA (yellow plug) to SCART adaptor if you plan to view video from a camcorder on a European television set.
If you have something on VHS video tape, it's best to convert to DVD before traveling. (Conversions between PAL and NTSC can be done before burning.) Use a video capture card for recording the VHS into a digital file on your computer. Then with DVD-making software, burn the file to a blank DVD.
Note that to be playable on a television set using a connected DVD player, a burned disc must be in the native DVD format (same as Hollywood movies) with the "AUDIO_TS" and "VIDEO_TS" folders. If you burn a Windows media, Powerpoint, Quicktime, Adobe flash, etc. file to a DVD, it can only be played back on another computer. This may be totally inadequate for a presentation. Unless your company or organization is already equipped, locating a computer video projector in a foreign country can be a challenge. Traveling with one is not recommended either, as they are expensive, fragile, and somewhat bulky and heavy. Exception: many newer DVD players can play "JPG" still picture files as a slideshow. Some even have an SD card slot, so you can view your photos taken from a digital camera. Caution: NEVER computer-edit anything directly on a photo-media card (SD, CF, Sony memory stick, etc.) Copy it to the hard drive or a USB jump drive first, then edit.
If required, converting DVDs from one format to another (PAL, NTSC), can be done on a computer with a fast CPU, or you can get it done professionally. Allow plenty of time, as this can take many hours. Regular blank discs work fine for making copies of a foreign format, as it's all just a bunch of ones and zeros and no different than copying anything else. Copies can be made quickly, while conversions cannot.
 Stay safe
The electrical engineer's maxim: The smoke that escapes from a device or a component is its spirit without which it cannot work. In other words: if smoke rises from the device, then it's destroyed.
The first time you use electrical equipment on a voltage system you haven't used before, watch for excessive heat, strange smells, and smoke. This is especially true for those residing in countries with 120V (USA, Canada, Japan, etc.) visiting places with the higher voltage. Smoke is a sure sign your equipment cannot cope with the voltage system.
If your electrical equipment gets very hot, smells of burning (there is a distinct smell of electrically fried circuit boards) or starts to smoke, turn it off at the wall or the main switch immediately, then carefully unplug the equipment. Do not disconnect or unplug by just grabbing the smoking device, its plug or cord, and then unplugging it, as these parts are probably very hot, and the insulation could be melted or unsafe, which could result in electrocution.
You may find your expensive equipment has been fried and needs to be replaced because the wrong voltage was used. However, if the equipment only got hot and did not smoke or produce strange burning smells you may be lucky. Some older devices have fuses that you may be able to replace. New devices, such as gaming consoles, will trip a circuit breaker. Disconnect them from all power and leave them for 60 minutes or so, and the circuit breaker will normally reset.
Do not rely on fuses to protect your equipment. If a fuse does blow, you should have things checked by an electrician before using the suspect equipment again.
In Third World countries with frequent blackouts, it's not at all uncommon for a visitor to plug something in and have the power go out coincidentally. Always check the neighborhood first, before blaming the appliance or looking at the fuse/circuit breaker.