Are RCDs really the panacea for electrical safety? James Eade finds out the ‘residual’ outcome of the arguments for and against.
RCDs are sensitive devices and – as anyone who has had unwanted operation at the most inopportune moment would probably agree – sometimes they can appear just a bit too sensitive. So, as a protective measure to reduce the effects of electric shock, it would seem that they are pretty perfect. Or are we making assumptions that, as the old expression goes, could be the mother of all cock-ups?
RCDs work by measuring the current flowing down the line conductor and comparing it with the current flowing back down the neutral conductor, as shown in Figure 1. If you ever did A-Level physics, the laws of a chap called Kirchoff might ring some distant bells; in one of them he stated that the current flowing in a circuit is always going to be constant. Applying that to any circuit, the number of electrons coming out of the source of supply, such as a generator, must equal the number of electrons going back. How they get back is key, and we try to ensure that the return path is always down the neutral, not through us or something else. And that is the principle of the operation behind an RCD – it measures the current flowing in the line and neutral conductors which should be the same. If it isn’t, the residual current is going back via some other means, which is not what we want.
The line conductor is wrapped in a small coil to create a magnetic field. The neutral is also wrapped in a coil, so, as a result, the two magnetic fields are, or at least should be in normal operation, equal and opposite, creating no resultant field. As soon as the currents are imbalanced, one field is stronger than the other so a resultant field builds up, which is detected by a third ‘search’ coil. All three are wrapped around a little ferrite ring, which helps to concentrate the magnetic field and all of this, including the electronic switching mechanism, is encased in a plastic housing. The whole thing is called a Residual Current Device, or RCD.
What could possibly go wrong with that principle?
Limitation number 1 – overload.
So we can see that the RCD measures imbalance only. If you were to come along and hold the line and neutral conductors, nothing would happen. If the riggers were to drop a scaffolding bar onto a cable and cause a short circuit between line and neutral, nothing would happen. Nor would anything happen if a bit of kit develops a fault and causes an overload, such as a stalled motor.
Well, by ‘nothing would happen’ – I mean the protective device in the form of an RCD wouldn’t work. The cables would still overheat and probably start a fire, and in the first case you’d suffer an elongated electric shock that would kill you. The important thing to remember is that RCDs do not protect against overloads or overcurrents in this respect as all are – as far as the RCD is concerned – varying levels of load current.
Rule 1: No imbalance? No operation.
Limitation number 2 – neutral-CPC faults
If there is a connection between neutral and the circuit protective conductor, typically, and wrongly, called the ‘earth’ wire after the RCD, the return current would be shared between the neutral and CPC, so the RCD would not actually be measuring anything reliably. It would depend largely on the impedance of the neutral and the CPC as to how much current would flow down each. Given the nature of the distribution cables typically used at events, the impedances are likely to be similar and the current shared. Adding a typical human as an additional fault to earth would make little difference to this and possibly not cause operation of the RCD.
Rule 2: Operation not guaranteed with wiring faults. Use good equipment.
Limitation number 3 – unwanted operation.
Unwanted operation is caused by one of two things – a fault, or bad circuit design. If there’s a fault, then it’s not unwanted operation – it is very much desired operation, so the fault should be found and fixed.
Bad circuit design is the most common culprit. How many times have you been working on an event and a faulty bit of kit causes the RCD in the main distribution to operate before the 30mA device it’s connected to? There are strict parameters for the operation times of an RCD based on the level of residual current. Unless you choose RCDs with the right level of delay and put them in the right places in the distribution, you might end up with all sorts of random operation. This is one situation where people often, erroneously, disable RCD protection as they are worried about losing power to everything because of one faulty appliance. Correct selection and placement of RCDs in this manner is referred to as providing ‘discrimination’ between devices, and time delays are achieved by using devices marked as ‘S-Type’ or ‘selective’.
Rule 3: Poor attention to discrimination may cause the wrong devices to operate.
Limitation number 4 – wrong type of RCD.
This is possibly the most insidious problem that few are aware of. By the ‘wrong type’ we are not referring to the residual operating current, known as the ‘I delta N’ rating from the symbol IΔn, but the design of the RCD itself. There are several types, the common ones denoted by the letters AC, A, B and F.
What makes them different is the way in which they detect the residual current imbalance. The most common, and hence cheapest, is the type AC. This only works on sinewave currents and any pulsing direct currents caused by harmonics, motor drives or dimmers for example will effectively saturate the magnetic field in the RCD. That prevents it from detecting an imbalance and therefore it won’t provide any protection. Type A is more reliable in detecting imbalances with such dirty supplies and types B and F even better.
As most temporary events have large quantities of switch mode power supplies, dimmers, motor drives for stage machinery and so on, type AC devices in the distribution are unlikely to provide reliable protection, if any, save for the very final circuits. The symbols for types A and AC are shown in Figure 2. These power quality issues can also cause random operation of RCDs, and even MCBs, even if there is little residual current.
Rule 4: Use the right RCD for the job.
Rule zero is ‘Never disable RCDs that are used as final circuit protection’; in other words the typical non-delayed 30mA device. Very rarely it may be permitted to disable RCDs providing protection of a distribution after a thorough bespoke risk assessment for that individual situation, not a standard tick-box form. This is a contentious issue and guidance is given in the IET Guidebook on temporary power systems. As a rule though, no RCD should be disabled unless you can really prove it is safe to disable what is in effect a life safety measure.
RCDs are brilliant devices that provide a lot of help in situations where the usual protective measures in a temporary distribution would not provide ideal protection to reduce the risks of electric shock – as is common at temporary events.
RCDs though are not, absolutely not, a panacea for poor design.
Please also remember that this is a potted guide to give a bit of background around RCDs – correct selection and application is quite a detailed process and needs careful thought by someone suitably skilled.
Nomenclature of some common devices:
Residual Current Breaker with Overload. This device is an ordinary RCD with a circuit breaker to protect against overcurrents, such as a short-circuit, built into the same unit.
Residual Current Device.A devices that disconnects a circuit once a specified residual current is attained.
Socket-outlet incorporating aResidual Current Device. This is, typically, an ordinary 13A domestic socket with an RCD built into the workings.
Residual Current Circuit Breaker – RCD and RCCB are the same thing and used interchangeably.
Residual Current Monitor. This is a device that has the sensing part of an RCD, but not the contacts for disconnecting a circuit. It just measures the residual current and will operate a signalling contact to drive a relay, sounder, contactor or other device when a preset level is reached. It usually allows the residual current alarm level to be adjusted, and has an adjustable time delay for activating the signal contact.
James Eade is a Chartered Engineer with over 20 years experience working in the event industry, including theatre, festivals, tours and corporate events. He advises several trade associations and represents the industry on various British Standard Committees, including those for BS 7671 and BS 7909. James is also the author of the guidebook on temporary power published by the Institute of Engineering Technology. For more information visit www.eade.uk.com