The common safety switch, as found in homes, is effective in protecting against dangerous leakage current but is generally unsuitable for many industrial applications. Phil Kreveld explains why.

Residual current devices (RCDs), also known as earth leakage circuit breakers (ELCBs), are designed to protect against dangerous touch voltages stemming from leakage current to earth.

However, effective operation depends on the nature of the devices and the circuits they monitor.

The protective role of RCDs is of relevance to multiple earth neutral (MEN) reticulation systems. These are termed TT systems internationally.

The other reticulation systems are TN and IT – respectively for a single earthing point for the neutral at the transformer, and for unearthed supply (except at the consumer’s premises).

IT supply initially requires an insulation resistance measuring device to report (and possibly interrupt supply should an earth fault occur), but the initial earth fault will not pose an immediate problem.

In the case of TN, an earth fault will cause sufficient current to flow for the operation of an overload circuit breaker.

The MEN system has the feature of restricting voltage build-up on neutrals. However, an earth fault exhibits the sharing of current paths between neutral and earth, thus requiring a differential current device for protection purposes.

RCDs can be divided into ‘supply’ and ‘supply independent’ types, as well as into those with entirely electronic or electromagnetic principles of operation, that is, CT classes.

Note that the existence of a CT does not rule out electronics (for example, some form of filtering) or the need for external voltage supply. The CT type is among the most commonly available RCDs for domestic applications.

The principle of operation is well understood: basically any leakage current seeking an earth return path bypasses the neutral wholly or in part so that the active and neutral current don’t cancel each other out.

A current transformer encircling active and neutral will not have an output signal when active and neutral are equal in value and opposed in direction. The tripping mechanism is obviously important and can make a substantial difference to the operation of an RCD.

However, the problems that can occur with RCDs – such as nuisance tripping or no tripping when they should – are related to detailed protected circuit considerations and to the properties of the current transformer magnetic circuit.

The operating principle of an electromagnetic RCD is shown in Figure 1a and a typical hysteresis curve of the magnetic core in 1b.

The vertical axis of the curve shows magnetising force in amp-turns, and the horizontal axis shows the magnetic flux density or flux (not including fringing flux, which shows up in equivalent circuits for such a device as leakage inductance).

All of this is boring stuff, yet it is important to understand the limitations of the device.

With the occurrence of unbalanced AC currents passing through the core, a signal in the secondary winding is generated equal to N d??/dt, where N is the number of turns in the secondary encircling the core flux ??, with d??/dt being the rate of change of flux.

Purely DC currents have no effect on transformer operation – that is to say, DC components that cancel each other out. Net DC current can, if large enough, cause saturation of the core.

The situation is different when, in effect, there is a DC bias current, as can occur in a rectifier circuit.

Take another look at Figure 1c. The AC waveform current (flowing as an unbalanced current) provides the full sweep of magnetising force, and therefore flux change.

The half-wave rectified current, on the other hand, provides only a partial excursion of magnetising force and flux, and a reduced voltage in the secondary winding.

This makes the average RCD problematic in operation when rectified waveforms are involved.

The operation of an electromagnetic RCD on DC component current requires the use of a ‘flat’ hysteresis magnetic circuit as shown in Figure 1d.

As can be seen by comparing the two hysteresis curves, the one specifi cally for the rectifi ed wave form requires a much smaller magnetising force in order to produce a significant fl ux change. However, the fl atter hysteresis curve also carries a disadvantage – that of possibly quicker core saturation.

The selection of core material is therefore very important in the construction of industrial RCDs, which typically are employed in circuits carrying not only distorted current but possible DC component fault currents.

These RCDs are distinguished by the type of circuit they are used in fullbridge, single-phase rectifi ers; SCRfi red power control; or single and three-phase inverters.

The various types are divided into classes AC, A and B. The AC variety is only suitable for AC circuits only, whereas the A type is applicable to AC and pulsed DC (rectifi ers, etc).

The B class is virtually universally applicable, including for pure DC (it can be used to protect solar panels).

DC relays can be divided into DC current sensing and voltage sensing. Current sensing makes use of a summation circuit based on a Hall-type sensor with both the positive and the negative bus currents as inputs.

In voltage sensing, a resistive divider circuit has to be part of the relay. When an earth fault occurs, there is a bypass of one of the resistors, and a voltage differential occurs that can actuate a relay via an operational amplifier.

These types of relay are used for protecting uninterruptible power supplies and, in general, balanced DC circuits such as DC links in variablespeed drives, etc, where either a positive or negative bus ground fault can occur.

Circuits

Reverting to the basic example of an active and one neutral, earth leakage detection is simple.

In industrial situations life is more complicated in part because circuit diagrams showing all revisions are often not available.

It is possible, for example, to have split neutrals, in which case unexpected tripping of an RCD can take place. This is not ‘nuisance tripping’, because the RCD is simply doing its job (that of detecting unbalance between active and neutral).

When power electronics and other equipment (fl uoros, switch-mode power supplies) are part of the circuit a new set of problems can affect the operation of RCDs.

As mentioned, RCDs have a problem with DC current. A phase to earth fault, even if high resistance, in the case of a converter or inverter will result in a unidirectional current.

The hysteresis curve in Figure 2a is of a high saturation core, and by means of an adequate number of secondary turns there is enough voltage created on its output to operate a relay.

The schematics in Figure 2b show typical power semi-conductor circuits subject to three classes of core characteristics.

Three-phase circuits present problems, in particular if harmonics are present – and when aren’t they?

The triplen harmonic currents (third and ninth in particular) are the so-called zero sequence harmonics, which fl ow in-phase and triple in value in the neutral of a four-wire system.

By rights, a CT core encircling all conductors will have a zero net flux, but the problem is in accessing the neutral for loads that do not have triplen currents. They do not contribute to the neutral current except for any unbalance between the phases.

The positioning of the RCDs can become a very difficult – if not impossible – task if there is no accurate reticulation drawing so that split neutrals are possible. Zero sequence currents can now cause real problems.

In general, harmonics are a problem. Mitigating their effects requires such solutions as chokes (thus increasing circuit impedance), harmonic traps and filters.

The problem with traps and filters is the use of capacitors, which provide not only filtering between phases but also between phase and neutral (and therefore earth, in effect).

Some filters are provided with low leakage current components, specifically for medical purposes. However, for earth leakage relays to operate reliably and avoid subjecting circuits to nuisance tripping, some form of filtering out unbalance signals in the CT secondary caused by the main circuit filters is necessary.

Quite apart from filter effects there is conductor capacitance, which in general is unbalanced.

In a variable-speed drive, the inverter section operating with a chopping frequency of, say, 4kHz will encounter unbalance between phases and neutral thus giving rise to higher frequencies earth loop current.

Likewise, transients originating upstream can result in unbalanced earth loop pulses and thus cause nuisance tripping.

The selection of RCDs for variablespeed drives, and other power electronics where EMC filters might be employed, requires great care.

In general, for the protection of a motor circuit powered by an inverter, an A type should be selected. The relay provides protection against a phase to earth fault for the inverter and for the motor.

Note that the existence of highfrequency earth loop current returning via motor bearings can cause bearing failure. An RCD will not protect in that situation unless it is capable of tripping on high-frequency current components.

Mining applications

The hazardous situation in a particular mine site cable pit would have been revealed by measuring the earth loop impedance.

The cable was connected to a 33kV/415V transformer and supplied a switchboard some distance away.

The cable had become damaged, and a miner working in the cable pit noticed that the surrounding sand was very warm. As he was leaving the pit he placed a steel-handled shovel against the metallic rim of the pit and received a shock in excess of 100V.

There was only primary winding fusing, and fuses did not blow. An obvious remedy in such instances is earth leakage protection.

Earth leakage relays have to be tailored to mining environments. In coal mining, relays must conform to AS2081.3. Such relays are used in earth fault limited systems. There are two types: core-balance and seriesneutral relays.

The core-balance type needs to be installed properly to minimise nuisance tripping. The smallest toroid that can encircle the cable should be chosen. If a cable passes through a large window toroid, there may well be a difference in leakage reactance of the three phases, thus increasing the probability of nuisance tripping.

Proximity to DC fields should be avoided, and other cables should be kept away from the toroid.

The series-neutral relay utilises a toroid with only the star-point earth conductor passing through it. In coal mining, the use of earth lockout relays is mandated (AS/NZS2081.4). Before energising the connected equipment the individual phase-earth loops are tested for loop resistance. If these tests are satisfactory, the interlocked circuit breaker closes.

Co-ordination

Time delay in the operation of earth leakage breakers is of importance, not only in terms of co-ordination of a sequence of circuits with differing protection levels but also to avoid equipment problems.

Zero-sequence currents – for example, third harmonics – and charging currents for long cables in the absence of a neutral connection have only an earth return path. In the case of charging currents, the transient effect can last for tens of cycles, and there should be a sufficient delay in the operation of the relay to cover this period.

Harmonics pose another problem and will be encountered particularly in plant with variable-speed AC drives.

Zero-sequence currents can in effect defeat the operation of earth leakage breakers. To provide discrimination, time delays have to be incorporated. Relays with time delay carry the ‘S’ designation.

Figure 3 shows a typical industrial installation and the RCD relays that might be employed to protect the entire installation, while taking account of the various earth path resistances encountered.

Ideally (that is, without taking nuisance tripping into account), a graded tripping pattern is established, with the main feeder relays tripping last.

Conclusion

RCDs are only a part of the overall protection scheme for a commercial, industrial or mining installations.

In planning the specifications, and where to install them, the line diagram of the installation should be checked against the physical locations of subdistribution boards, and the type of loads connected to these so that the appropriate type (AC, A, B) is selected.

It is then necessary to verify earth loop impedances for individual circuits. If these impedances are too large, tripping will not occur until touch voltages have built well beyond the 50V AC level allowed.