Drives and the role of the RCD
26 October 2010
A residual current device (RCD) that doesn’t work is worse than no RCD at all, as it lulls the user into a false sense of security. The purpose of the RCD is to provide safe circuit isolation when leakage to earth is detected. But in applications where earth leakage is part of the normal installation, such as ac drives and EMC filters, this operating principle is compromised. The RCD may then fail to operate properly and can disrupt operations as well as putting users at risk. Geoff Brown offers
The RCD can be a valuable piece of equipment when it comes to preventing electric shock. However, to be of value, the user has to be able to trust the device to work at all times. Unfortunately, this cannot always be taken for granted. A recent study by the Danish equivalent of the UK’s Health and Safety Executive, Sikkerhetsstyrelsen, showed that seven percent of installed RCDs it surveyed were not working properly.
One area where the installation of RCDs is particularly problematic is where it feeds devices for which earth leakage is inherent to the design; EMC filters and ac drives are good examples. In this case, the RCD may fail to operate or the power supply may be interrupted randomly. This tends to happen when the RCD used is not compatible with the protected equipment.
Some years ago, my company installed some new drives at the site of a major UK automotive manufacturer. The circuit already had some existing drives, as well as some sockets protected by RCDs. After the installation, the RCDs were found not to be working, having always been tested when the drives were stopped. We investigated the matter and discovered that the RCDs had actually never worked, despite being tested once a week, which was very disturbing news to the management; somebody might have been killed as the result of an electric shock.
Checking the balance
The RCD compares the balance between the phases using a magnetic coil to detect any imbalance. Such an imbalance is sometimes caused by current leakage through the body of a person who is grounded and accidentally touches the energized part of the circuit. Leakage to earth will cause imbalance between the phases and the RCD will immediately interrupt the circuit. If the breaker does not work, a shock may be delivered that can cause serious injury or death.
When earth leakage is inherent to the design of the device, the detection circuit may not work properly. Problems with EMC filters can sometimes be remedied by selecting a low leakage filter; with ac drives, however, the issue gets a little more complicated. The RCD may be sensitive to the dc component – most likely from the drive’s rectifier when the supply is unbalanced. This can cause the magnetic circuit (the part of the RCD that provides the safety element) to be become saturated, which can decrease its sensitivity and may prevent it from operating correctly. This can be addressed by using a ‘Type B’ RCD.
Another potential problem is nuisance tripping at start-up, or even during normal operation. If the individual phase contacts of a switch do not connect at the same time, the RCD may detect phase imbalance. When the drive initially starts up, it only draws power on one phase. This only lasts for about half a second, but this is plenty of time for the RCD to trip. This could happen all of the time, or occasionally. If it works some of the time, it may be that the current is close to the trip level and the RCD may stay connected after a couple of resets.
If the drive trips during operation, this may be caused by switching operations or network disturbance. One source that can upset the balance between the phases is the leakage current in the screen of the motor cable. Due to the operating principle of the ac drive, there is always some leakage or common mode current in the supply, main and motor circuits. Longer motor cables have more of this leakage current than short ones. This current should be returned to earth using separate earth bonding from the motor and the driven machine via the drive, not returned to the drive by a common path with the measuring circuit.
RCD or no RCD?
The most obvious way to prevent RCD problems in drive installations is perhaps not to use an RCD in the first place, but instead rely on other safety devices, such as fuses, and by ensuring that good earth bonding practice is followed. Unfortunately, drive installations are overlooked by the UK wiring regulations, which suggest that the RCD is always the preferred form of circuit protection.
Alternatively, there are modern drive-compatible RCDs equipped with filters that prevent the device from detecting some high frequency currents, reducing the number of nuisance trips.
The RCD must also have high enough trip level. To find out the required trip level, a simple common mode current measurement over the supply phases can be calculated. Alternatively, the value during power-up and continuous operation can be tested.
One RCD per drive
It is also important that each drive has a dedicated RCD that is not protecting any other equipment. To improve stability, the RCD can be by-passed during start-up to prevent tripping, providing that electrical safety can be ensured some other way.
If the drive is installed in the second environment, it may also be possible to remove EMC filters if these are causing problems, or to remove the grounding screws of the EMC filters. However, this will increase high frequency emissions, which may then become higher than local regulations allow. In this case an EMC plan should be drawn up outlining how these issues can be dealt with.
Even if the RCD itself works properly, it can still fail to protect the user of an ac drive, as the drive’s dc link capacitors can hold a substantial amount of energy in the zone supposedly protected by the RCD. This energy could still deliver a powerful shock, even if the circuit is interrupted by the RCD.
Although tripping can be annoying, it is important not to take any short-cuts that compromise personal safety. Always ensure safety and protection in your ac drive installation by providing adequate grounding, correct fuses and breakers in the power supply to the VSD as appropriate, insulation and separation of circuits.
Each case should be studied on its own merits. If your drive and your RCD don’t seem to be getting on, you may need to look at some of the remedies suggested in this article. But it is perfectly possible that your ac drive installation will provide years of trouble-free service, protected by a modern, drive-compatible RCD with suitable filtering.
Geoff Brown is senior consultant, drives and motors, at ABB
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