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Safety matters - don't be tripped up by RCD type

01 May 1999

In the first of an occasional series of articles on safety issues, Andrew Cooper offers advice on the selection of the correct type of residual current device for inverter drive installations In electrical installations, including inverter drives, fuses do not always provide adequate protection against shock hazard or fire.

To increase the level of protection for personnel and property, residual current devices (RCDs) - also known as earth leakage circuit breakers - have been developed, and these are now widely used throughout industry, agriculture and even in domestic environments. An RCD is shown in diagrammatic form in Figure 1.

It embraces all live conductors, L1, L2, L3 and N, except the protective conductors PE and PEN. There are three basic types: AC, A and B (Figure 1 shows Type B), and it is important that the right type is installed, particularly where ac inverter drives are concerned. Figure 2 shows the fault currents that can be detected by the three RCD types.

Type AC, an early device, can only detect ac fault currents, whereas Type A - the most commonly used RCD today - can detect ac and pulsating dc fault currents, provided the dc fault currents cross or touch zero at least once in every full mains voltage cycle. Type A has only one coil (W1 in Figure 1).

A relatively new RCD, Type B, has two coils, but in most other respects it is similar to Type A - albeit, a little more expensive. It can detect ac, pulsating dc and smooth dc fault currents. Choosing the right type Bearing A Type AC RCD should never be used with inverters, since the fault currents experienced with these devices are never clean ac fault currents. A single phase inverter has a B4 rectifier, as shown in Figure 3.

The fault current is a combination of ac and pulsating dc, the latter always touching zero in between two pulses. The RCD coil (W1 in Figure 1) will therefore not saturate due to the dc content in the fault current, so a Type A device can be used. Three phase inverters, on the other hand, have a B6 rectifier (Figure 3) and the fault current does not cross zero because all the diodes are never off at the same time.

The dc content in the fault current from a B6 rectifier is thus likely to cause W1 to saturate, and Type B is more appropriately applied. This has two monitoring circuits, one of which is designed for monitoring dc fault currents (W2 and E in Figure 1). RCD coil W2 will not saturate due to the electrical circuit to which it is connected.

The author's company recommends that a 300mA Type B RCD be used in conjunction with its three-phase inverter range. If an RFI filter is mounted in the drive and either the switch of the RCD or a manually operated switch is used to connect the drive to mains voltage, a minimum 40ms time delay is essential.

If, however, no RFI filter is mounted or a contactor is used for mains connection, no time delay is required. Leading manufacturers of Type B RCDs can provide the necessary built-in time delays. The reason why the RFI filter may cause problems at start-up is that the simultaneous cut-in of all three phases will result in the earth leakage current from the capacitors being close to zero.

If a phase is cut in before the other two phases, the resulting current will exceed the trip level of the RCD. If a manual switch is cut in very fast, the time delay between the three phases should be short enough, but users should be prepared for occasional trips. The Type B device is now seen as something of a panacea, but users should be aware that not all installations require this level of (expensive) sophistication.

A single phase inverter will be adequately served by a Type A device, for example, and there is no requirement for a time delay regardless if RFI filters are mounted or not. Moreover, regulations vary from country to country, application to application, and it is advisable to investigate local requirements and legislation concerning the use of RCDs in order to keep installation costs to a minimum.

Andrew Cooper is with Danfoss 

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