Difference between revisions of "RCD"
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Revision as of 00:33, 22 November 2008
A Residual Current Device or RCD is a circuit protective device designed to achieve two important results:
- To protect users from serious electric shock.
- To protect circuits from sustained earth faults that may occur where the normal operation of traditional protective devices like Fuses, and MCBs does not function correctly because the earth loop impedance is too high.
All RCDs will achieve (2), and this in turn also helps protect users from shocks as a result of Indirect contact. For protection against direct contact shocks however, only the versions with 30mA or more sensitive trip thresholds will achieve this effectively.
What Does it do?
A RCD detects fault conditions where current flowing into a circuit is finding unexpected and unintended routes back out of it. These include the situations were a person is receiving an electric shock from the circuit, or when current is being conducted away to earth for some other reason. When any of these situations are detected, it automatically disconnects the circuit.
For users, this gives greatly enhanced shock protection from both direct contact (i.e. contact with an exposed live wire - say touching a flex you have just damaged with a power tool), and indirect contact (e.g. when the metal casing of an appliance becomes live due to an internal fault).
RCD protection is particularly important to protect users in high risk locations where they may be more susceptible to electric shock; such as bathrooms, pool areas, saunas, or simply when they may be using power tools or appliances outdoors (basically anywhere the user can be expected to be wet, barefoot, or in good contact with earth).
What does it not do?
The most commonly used type of RCD does not offer any overcurrent protection, and so it will not clear short circuits, or faults that result in an appliance drawing excessive current (there is a type that does offer overcurrent protection as well - the RCBO (see later)).
When using an RCD protected supply to feed a power tool, they will offer no protection should you make contact with another live, non-protected, circuit with the tool. So if you drill into a non-RCD protected live cable, the RCD powering the drill will not detect any current flowing from wall cable to drill body, and from there to the user. Neither can it switch this current off. (just as well most drills these days are double insulated)!
While RCDs act on the majority of shock situations, they won't protect you from all of them. Situations they don't protect against:
- a shock received by making contact with both live and neutral
- When a RCD does operate, it can take up to two cycles of the mains (i.e. 40 msec or even more for low leakage situations).
- RCDs don't limit shock current. They have no means to do so. They limit shock time and so the total energy imparted in the shock, which is a real plus, but this does not make shocks entirely safe.
- They don't act at all on shock currents below their trip threshold.
- RCDs may fail to operate on rectified shock currents.
- Higher trip current RCDs fail to act on lethal currents below their tripping point
- RCDs don't operate on shocks from transformer isolated high voltage supplies in appliances. Examples of such appliances would be TVs, microwaves, etc.
- RCDs don't act on shocks from transformer isolated shaver sockets in bathrooms.
- A plug-in RCD doesn't protect against drilling into a live wire
- RCDs are not perfect devices, and sometimes fail to function entirely.
Also the degree of shock protection RCDs provide does not make shocks safe by any means:
In summary, RCDs do significantly reduce shock risk, but do not eliminate it.
How does it work?
RCDs are current balance devices. They measure any current imbalance in the flow in and out of a circuit or appliance via its Live and Neutral conductors (or on three phase circuits, the combined sum of currents in all phases and neutral). Should the current imbalance exceed the tripping threshold for the device, it will activate and disconnect the circuit.
For more information see Wikipedia RCD entry
Where are they used?
In the past RCDs were mandated for protection of any circuit that your could reasonably expect to power portable equipment that could be used outside. So this would have usually include at least the downstairs socket circuits, plus any socket circuits in outbuildings, garages etc. The current 17th edition of the wiring regs now expand this requirement to basically cover all circuits where the cables for it are concealed and not otherwise protected (i.e. by burial >=50mm from the surface, or via earthed metal sheathing on conduit, or using cables such as MICC or SWA). Note this now means that even lighting circuits will be routinely protected by RCDs. This places a greater emphasis on careful design to ensure good discrimination between circuits to minimise the knock on effects of faults.
For properties that are not provided with a main earth connection by their electricity supplier (often those supplied via overhead wires), a local earth stake or grid is usually used to provide an earth connection. Since it is not usually possible to achieve a low enough resistance to earth with this technique to allow correct operation of circuit protective devices (fuses or MCBs), RCD protection is mandated for all circuits in these TT systems (regardless of the level of cable protection used).
In the past, to maintain discrimination between different classes of circuit, and prevent the problems associated with having a "whole house RCD", several RCD devices were used. A device with a 30mA trip threshold will protect the relevant socket circuits, and a higher 100mA trip threshold device protects all the other circuits. TT will typically still retain a similar configuration, but is more likely to use multiple 30mA RCDs now.
See Earthing Arrangements for more information.
Types of RCD
There are a number of different types of RCD available with different form factors and technical ratings. Hence you need to select the right unit for the job.
|Integrated plug adaptor RCD||There are a range of RCDs that are built into plugs and sockets. These are designed to offer enhanced shock protection to either an individual device, or small number of devices. Typically found on extension leads.
There are also some fixed wiring sockets that include RCD protection, again designed to provide a safer connection point for certain categories of appliance.
Note that some of these plug in devices also incorporate a "No Volt Release" (NVR) function. This in effect switches off on loss of power as well as on detection of a fault. These hence require a manual reset after a power interruption. This style of operation is often preferable when protecting power tools, since they prevent the operator receiving a nasty surprise when the reason for the apparent "trip" was in fact a short power cut or the plug coming out, and they forgot to turn the tool off!
|Integrated RCD Spur||A RCD integrated into a spur connection unit. Designed to provide individual RCD protection any fixed equipment that has a high electrical shock risk (e.g. a pool/bath hoist for disabled access).
May also include NVR function as above.
|Standard DIN rail mounting||This is a modular device in a standard form factor that is designed to be used in electrical enclosures such as consumer units and other similar enclosures. These RCDs typically occupy two module widths (i.e. the space taken by two MCBs), and can be used to power one or more circuits. They also meet the isolation requirements for a main switch, so they can be used as a direct replacement for the double pole incomer switch on a standard consumer unit.|
|DIN rail mounting RCBO||A Residual current Circuit Breaker with Overcurrent protection (RCBO). These are effectively the combination of a Miniature Circuit Breaker and a RCD in a single unit. Hence they provide RCD functionality and also overcurrent protection. Unlike normal MCBs they include a separate connection to allow the neutral of the circuit to return first to the RCBO, and then be connected to the non RCD neutral busbar of the consumer unit.
These are very handy devices since they ensure good discrimination should they trip - only the affected circuit is taken out of action. The disadvantage of RCBOs is firstly they are expensive (especially if you need to protect a number of circuits), and secondly many of them are physically wider than a standard MCB. Hence they require the use of a consumer unit with more "ways".
(Note the unit pictured is a single module width version. These do not require more width than a standard MCB, but will only fit in consumer units with enough height to accommodate their extra length)
Electrical and Trip Characteristics
|Rated Current||This is the maximum current the device is rated to carry. Devices in DIN rail mounts are commonly available in 40A, 63A and 80A ratings.|
|Number of poles||RCDs are available for protecting both three phase and single phase circuits. (Three phase devices being typically twice the width of single phase ones)|
|Trip threshold or sensitivity||This is the maximum current imbalance that will be tolerated without the trip mechanism being activated. In reality the devices specifications are usually scoped such that the device will trip on 66% of the rated trip current (so as little as 20mA may be required to trip a 30mA device).
Common trip thresholds include:
Devices with thresholds of 30mA or less will offer good direct contact shock protection. Higher trip threshold devices are only suitable for providing circuit protection from high earth fault loop impedances (and hence providing indirect contact shock protection).
Although RCDs can trip below their stated threshold, to meet required standards RCDs should not trip on leakage currents any less than half their threshold. They must also trip within 40 msec for any trip current that is five or more times the trip threshold (many will perform better than these minimum performance requirements)
|Trip time||General purpose RCDs (sometimes marked with a "G" suffix) are designed to trip as soon as possible after a trip condition is detected, and usually within two cycles of the mains (40 msec for UK 50Hz supplies).
There are also time delayed types that are designed to trip only after exposure to a trip fault condition that lasts longer than a pre-set delay (typically two seconds). The time delayed type (often denoted with a "S" suffix) are particularly useful where it is required to cascade RCDs. The time delay maintains discrimination between the cascaded devices so that the downstream one closest to the fault trips first.
|Overcurrent Trip Threshold||Only specified for RCBOs. The range of overcurrent thresholds available (typically 6A, 16A, 20A, 32A, and 40A) is similar to that for MCBs. In many cases a RCBO can be used as a drop in replacement for a MCB.|
Earth Leakage Circuit Breakers were the forerunner of RCDs. There were two basic types: the most common was described as a Voltage Operated ELCB, which in theory tripped when it detected a large voltage rise on the main earth conductor, however in reality since the main earth conductor was connected through it, it could also be regarded as behaving as a current operated device - not unlike a modern RCD, but detecting current flow through the earth conductor rather than imbalance between phase and neutral.
There was also what was described as a current operated ELCB. This is simply an old name for what we now call an RCD.
A "voltage operated" ELCB:
Old voltage operated circuit breaker connected in the meter tails before a consumer unit, with the main earth connection from the consumer unit fed back through the ELCB.
Residual Current Circuit Breaker (RCCB). This is a term that does not not appear in the current wiring regs, and does not have a consistent definition or usage. Some manufacturers use it to differentiate RCDs without overcurrent protection from those with it (i.e. RCBOs).
A Nuisance trip is an unexpected operation of a RCD that does not appear to be related to an immediately obvious fault. There can be many reasons that these trips occur, some indicate that there is a latent problem with the electrical installation, some may indicate the presence of a serious but as yet unobserved fault, and others may be the result of a minor fault that in itself poses little or no risk.
Tracing the cause of nuisance tripping can prove to be very difficult and time consuming. This section will attempt to provide some guidelines to help.
What causes nuisance trips?
Excess earth leakage
The RCDs operating principle is to measure the current imbalance between that flowing into and out of a circuit down live and neutral wires. In an ideal world the current difference would be zero, however in the real world there are a various different types of equipment that will legitimately have a small amount of leakage to earth, even operating normally. If the RCD is protecting too many such devices then it is possible that the cumulative result of all these small leakages will be enough to either
- trip the RCD
- or by passing most of the RCD's trip threshold current, make the RCD excessively sensitive to any additional leakage currents
|Electronic equipment with mains input filters||Much modern electronic equipment includes a mains input filter designed to stop electrical noise being passed in or out of the equipment via its mains lead. These typically include a pair of small capacitors, one connected between the live and earth, and the other between the neutral and earth wires of the incoming mains lead. The capacitor values will be chosen such that they conduct well at the typical noise frequencies that are intended to be filtered. However a tiny amount of current flow will occur at mains frequency, and this results in leakage to earth. It is also worth noting that the filter circuit is designed to snub noise by shorting it to earth. Hence the noise itself can also contribute to the total leakage current seen by the RCD.|
|Heater elements||Many heater elements that are designed to heat water (kettles, immersion heaters in hot water cylinders, or washing machines, ovens, grills etc) use a mineral insulation that is hygroscopic. Hence when left unused for a time they can absorb a small quality of water into the insulation. Since water is electrically conductive to some degree this results in a small amount of leakage to the outer (earthed) metal case work of the heater element. This type of leakage normally poses little or no risk, but will frequently trip an RCD. One way to clear the problem is to run the heater and drive off the moisture. However it is possible to enter a catch 22 situation here, where the RCD prevents the heater from being run.|
|Dampness||Any device that handles water and electricity will be vulnerable to dampness getting into electrical connections or wiring harnesses. This can result in short term high levels of leakage that mysteriously vanish later (as the affected item dries out). Even condensation forming in equipment can cause this problem.|
Split water heater elements. These cause gross earth leakage, and conduct it directly through the water being heated. Contrary to what we were taught about electricity and water in primary school, this does not cause electrocution in practice. Split elements can however cause overcurrent leading to overheating of electrical accessories.
The effect of high natural leakage currents can be to consume most of the trip current "budget" of the RCD, leaving it very close to its tripping point. Once this situation has been reached, then even minor changes in circuit environment or use can result in trips. These include:
|Switch on surges||When devices with mains input filters are switched on, there will be a brief period where its filter capacitors are "charging up" and passing more leakage than usual. This can be one cause of trips. Also some devices will absorb a large "inrush" of current when first turned on. This can itself generate lots of harmonic noise that is then dissipated to earth by the filter capacitors (something similar can in some cases happen on switch off too).|
|Changes in humidity||A simple thing like a damp day can be enough to slightly lower the effectiveness of insulation used on cables and in equipment, resulting in more leakage. Electrical installations outside, or in outbuildings are particularly vulnerable to the effects of moisture.|
|More appliances in use than normal||Using more appliances than normal or an infrequently experienced combination of them may push the leakage over the limit.|
Whole House RCD
This is a deprecated way of installing an RCD such that a single low trip threshold device (typically 30mA) protects all the circuits in a property. While counter to the advice given in the present wiring regulations. installations of this type are still commonly found. Whole house RCDs are very vulnerable to nuisance trips, and any such trips remove all power to the property.
|Neutral to Earth shorts||A particularly problematic fault is a short between neutral and earth on a circuit. Since Neutral and earth are nominally going to be at a similar potential (especially in buildings with TN-C-S / PME earthing (see Earthing Arrangements)). You can arrive at a situation where the current flow between neutral and earth is lower than the trip threshold of the RCD some of the time, however once the neutral current reaches a high enough level, its potential will be "pulled" away from that of the earth, and you get increased leakage current flow which may cause a trip. Needless to say this threshold will often be reached during transient current peaks caused by equipment being switched on or off.|
|Insulation breakdown or damage||As cables and wires age, their insulation can become less effective. This is especially true if you live in an old property that still has rubber insulated cables. Humidity will also reduce insulation effectiveness.|
One obvious possibility (and often overlooked) is that the RCD itself is actually faulty and not tripping at the correct current. A RCD that refuses to reset even when all output connections are removed is an obvious candidate for landfill. Swapping the device with a known good one, or using a RCD test facility as described elsewhere on this page are other ways of finding faulty RCDs.
Many RCDs include a "test" button that verifies the unit functions. This simulates a imbalance current internally, which causes the device to operate. Note however that because the test current may be several times the trip threshold, it does not test if the trip threshold has drifted too low or the mechanism has become slow - only that the trip detection and basic mechanics still work.
How to locate the cause of nuisance trips
There are a number of empirical tests or experiments that you can try to narrow down the source of the problem. We cover some here. The first job is to identify which circuits the RCD is protecting. There is no need to concentrate efforts on examining circuits that are not connected and hence can not be affecting the outcome!
|Turn off circuits in turn||You may be able to identify which circuit is causing the problem by isolating circuits in turn, and seeing which prevents the trip from reoccurring.|
|Remove appliances from suspect circuits||Disconnecting appliances from suspect circuits can let you identify if the fault is in an appliance (the most common situation) or the circuits fixed wiring. If you still get trips with everything disconnected then you may have a wiring fault.
If it looks like appliances are to blame, you can apply the "binary chop" principle to narrow down the field quickly - i.e. unplug half of them and see what happens. If it still trips you know in which half the dodgy appliance probably is. The carry on in the same way - halving the list of remaining suspects, until you get close to the answer. (This method isn't bulletproof with RCDs.)
|Check the likely culprits||Identifying which appliances you have from the "high risk" categories listed above can help to take you to the cause of the trouble faster.|
|Identify coincidental factors||Check for any patterns and relationships between trips and other events. Do they occur only in damp weather, or only at certain times of day, or only when the freezer switches on, or the Central Heating. Pay particular attention to automated systems (timers, thermostats etc) that can be controlling significant bits of electrical equipment in your home without your manual intervention.|
|Introduce extra leakage||To correctly test the function and trip threshold of a RCD you need a specialist test meter (see section later on testing). However there are various home built devices that can help you to perform some simple tests. One of these is a leakage plug, which can aid finding problem circuits by letting you introduce a known amount of leakage into a circuit.
To make one you need a conventional 13A plug, fused with a 3A fuse, and and internal connection between earth and live made via a high value high power wire wound resistor. There are no other connections made to the plug externally, and the plug should be clearly labelled. Turning on each of the protected circuits one at a time, and using the leakage plug on it can help identify a circuit that leaks more than the others (since the combination of the plug and its leakage will trip the RCD on the high leakage circuit).
A set of plugs wired for different leakages will help you get an approximate idea of the leakage level caused by the circuit (and its appliances) itself (note the better RCD testers have a current ramp facility to better conduct this test):
(Don't leave the plug connected for too long, or its resistor will get hot!) Keep such plugs secured away from children for safety reasons.
This section deals with the particularly tricky problem of tracking down the causes of nuisance tripping that you have not been able to find by other methods.
For detailed tests on RCDs a specialist test meter is required such as this. An insulation resistance tester such as this may also be required to track down some of the more difficult to locate wiring faults.
However it is possible to carry out a good number of tests using more basic equipment such as a multimeter.
There are procedures that are described here that require opening your consumer unit and making temporary wiring alterations within it.
If you are not fully competent to do this, then please consult a technically skilled electrician.
DC Resistance tests
First ensure that power is switched off at the main switch. Ensure all appliances are disconnected from the circuit. These tests require that you disconnect the circuit under test from the consumer unit. In the case of a ring circuit remember to disconnect both legs of the ring. All tests are initially performed on the disconnected ends of the circuit.
There are a number of basic tests that you can do that will identify a great many of the fixed wiring faults that can cause nuisance tripping.
|Live Neutral Resistance||The first test is a simple resistance test between live and neutral. This test should be done using the highest resistance range on your multimeter. Normally with all the appliances disconnected you would expect to see an open circuit between live and neutral. If this is not the case then you either have something still connected, or you have a serious insulation resistance problem.|
|Live Earth Resistance||This test should also indicate an open circuit with the multimeter on its highest resistance measuring range. Any non infinite reading here could be a direct indication of your problem. If you get a non infinite resistance reading, you may be able to track down the location of the fault by breaking the circuit up at strategic points (typically by disconnecting part of it at an accessory position).|
|Neutral Earth Resistance||Again this test ought to indicate infinite resistance. However it is possible that a very low resistance measurement could exist and yet the circuit still work some of the time (especially on systems with TN-C-S earthing). Unlike a low resistance reading on a Live to Earth test, this fault would not immediately trip a MCB or blow a fuse.
Tracing the location of the short or bridge can again be done using the segmenting procedure described above, and also by careful low resistance measurements made in conjunction with expected cable resistances as found in a Wire resistance table (or see the table in the IEE Wiring Regulations On Site Guide).
A typical cause of this type of fault, is where a concealed cable has been damaged by a fastening being driven through it. (so if any shelves or pictures have been hung recently, there is a good place to start looking).
If the DC resistance tests above fail to identify the cause of a circuit that is causing RCD tripping on its own (i.e. without the aid of the appliances usually connected to it). You may find that repeating the tests described using an insulation resistance tester will yield more information. Since the insulation resistance tester carries out the tests at much higher voltages than the multimeter (typically 500V) it will identify those few failures where the conduction path between a live conductor and earth is only visible at mains voltages.
Take care when performing these tests, it is possible to get a nasty shock off an insulation test meter!
Series earth current measurements
A test technique that can be quite handy for testing individual appliances, is to measure the actual current flow in its earth wire connection. To do this safely one needs to make up a suitable test lead to allow safe access to the earth connection. The best way to measure the current flowing in the earth wire is using a very high sensitivity clamp meter (see next section), since this leaves the earth wire connected directly to the appliance.
If one of these is not available, then it is possible to make a measurement using and AC current measurement range on a multimeter. The meter will have to be placed in series with the earth wire and any leakage current will pass through the meter and can be measured. It is important to note however that this carries a certain amount of risk since should there be (or occur) a fault in the equipment, the full fault current may try to pass through the meter. Most decent quality meters will respond by blowing an internal fuse, but some might melt, catch fire, or rare cases explode! If the meter fuse does blow it will have in effect opened the connection between the appliance and earth. This will mean its exposed metalwork may now be sat at mains voltage with no protective measure to cause a fuse or MCB to operate.
For these reasons
- you need to know how to handle such situations safely
- its not advisable to use this technique at the consumer unit for a whole circuit unless clamp meter is used.
High sensitivity clamp meters
Earth leakage clamp meters like this have recently become a popular way to detect earth leakage faults. These can be safely connected around either an earth wire to directly measure leakage to the equipment earth, or around live and neutral to directly read current imbalance. Since they require no physical connection to the appliance or circuit under test they are a much safer way to measure either leakage to earth or imbalance in the supply to a circuit or and appliance. Measuring the imbalance will help detect where the leakage is not occurring through the circuit or appliances own CPC but instead is finding an alternate path.
Mitigating the effects of nuisance trips
While it is possible to eliminate most causes of nuisance trips with careful system design and testing, it is always wise to design the system to allow for the possibility of it happening:
- Provide dedicated non RCD protected circuits [see note] for vulnerable equipment such as:
- Have as few circuits or devices as possible protected by the same RCD so that a trip impacts as few extraneous circuits as possible. The ultimate solution would use RCBOs for each circuit. Obviously expense has to be weighed against the implications of tripping.
- Use emergency lighting to backup any important lighting circuits that need to be RCD protected (i.e. on TT earthing systems). In particular these should include lighting for:
- Fire escape routes
- Near trip hazards or other difficult to navigate areas
- Near the consumer unit
- Consider using uninterruptible power supplies (UPS) to maintain running of critical equipment.
- Power failure alarms might also be an appropriate measure in some circumstances.
Note: With the advent of the 17th edition of the wiring regulations, one must comply with the requirement that any buried cables that don't have 30mA trip RCD protection, must still be adequately protected from physical damage. This can be achieved either via being buried at 50mm or greater depth in a wall, or with metallic earthed protection such as conduit or by using suitable metal sheathed cables like SWA, MICC etc. Note that new cable types are becoming available to help meet these requirements. Surface mounted cables may also be installed without additional RCD protection in some circumstances since it is assumed they are sufficiently visible to avoid accidental damage from drilling / nailing etc.
System design using RCDs
Some of the system design aspects of using RCDs to good effect are covered in the mitigation section above. However the following basic principles should also applied:
- Use split load consumer units, to allow circuits that do not benefit from RCD protection to be powered directly.
- Don't place too many circuits on the same RCD. In particular identify circuits that are likely to have high leakage (e.g. those containing lots of IT other electronic equipment).
- Where RCDs need to be cascaded, use time delayed types for the upstream device so that trips are contained close to the cause of the fault.
- Don't place circuits to outside electrics and outbuildings on the same RCD as protects the house circuits.
- Avoid placing high leakage devices on RCD protected circuits where possible.
- Design circuits such that the anticipated leakage is no more than 25% of the trip threshold. This will allow for later circuit extension.
- Ensure accessories and wiring are not placed in excessively damp environments.
- Don't use lower trip threshold devices that is appropriate for the level of risk present and protection sought.