Earth Leakage in Variable Speed Drives

In this article by Colin Hargis, chief engineer at Control Techniques, we look at earth leakage in variable speed drives.

AC variable speed drives (VSDs) generate earth leakage currents which are mainly harmless but can sometimes be high enough to cause nuisance tripping of RCDs (Residual Current Devices, earth leakage breakers etc.). Here we look at the reasons for the earth leakage current and possible remedial measures.

Simple electrical devices such as motors and heaters have no connection between the AC power circuit and the earth (PE) connection. The earth connection is there to keep the device safe in the event of a short circuit from the power circuit to the conductive structure. In normal use, there is a very small leakage current flowing to earth, caused by the small stray capacitance and resistive leakage of the power circuit to the earthed structure. This current is in the region of a few micro-amps and has no perceptible effect.

Most electronic equipment has some radio interference filtering, which usually requires capacitors between the power lines and earth. This then results in a leakage current through the capacitors to earth. For a single phase supply, the leakage current can easily be calculated from Ohm’s law and the reactance of the capacitor at the supply frequency.

Limits to leakage current for safety

For electrical equipment provided with an earth connection, referred to as “class 1 equipment”, and designed to be plugged into a mains outlet, the design has to be such as to keep the leakage current below 3.5 mA as laid down by safety standards. The idea is that even if the earth wire were to break and a person were to touch the unearthed equipment, the current of 3.5 mA would not harm them (although they would probably sense it and find it unpleasant). In safety tests what is measured is actually the “touch current” which is measured using a circuit which models the impedance and frequency response of the human body. In practice, leakage current and touch current are usually rather similar.

Where the equipment is permanently installed with a fixed earth connection which is strong enough to avoid accidental disconnection or breakage then this limit does not apply. That is why Control Techniques drives are supplied with either a PE terminal with capacity for a 10 mm2 earth wire or two terminals for two smaller earth wires.

Residual Current Devices (RCDs)

An RCD is a protection device which can be fitted to an electric circuit of a building and which trips to remove power if it senses excessive leakage current. It does this by measuring the total current in all power phases (L+N for a single phase, L1+L2+L3+N for three phases), which should ideally be zero – i.e. all current should circulate in the power wires and not flow to earth.

Figure 1, below, illustrates the arrangement for a single phase device. The most common trip setting is 30 mA, which is a level which is low enough to prevent harm to a person, but high enough to avoid frequent nuisance trips. There are many reasons for fitting an RCD, the most common is where portable equipment is being used in a place where there is a raised risk of electric shock – for example, a sharp electrical tool which might damage its own cable or a wet area.

Why are VSDs special?

The VSD contains an inverter which uses very fast switching of the output voltage and PWM to synthesise the required current waveform for the motor. Most modern power electronic equipment uses similar fast switching power devices, and to avoid the emission of radio frequency interference (electrical “noise”) it has to be provided with a radio interference filter.

The VSD is unusual because the switching output is not contained within the unit, but passes through the motor cable, which may be many meters long, to the motor. This means that there is a high stray capacitance to earth which drives a high radio frequency current and requires a particularly strong filter to prevent harmful emission into the mains supply. The filter needs to have unusually high values of capacitance to earth in order to be effective, especially with a long motor cable. Most of the filters offered for Control Techniques drives are designed to be used with up to 100m of motor cable.

Figure 2 illustrates the leakage current paths for a VSD with input filter. The capacitors themselves cause a certain leakage current at the supply frequency of 50/60 Hz. In addition, there is a high-frequency current flowing into the stray capacitance of the motor and cable. The capacitors have low impedance for the high frequencies and they divert the high-frequency current so that very little of it flows in the supply. That is their function as interference filter components. In return for this useful effect they generate some 50/60 Hz leakage, but because of the lower frequency, it is much smaller than the diverted high-frequency current.

Practical effects of VSD leakage current

The most common kind of drive system uses several drives and other electrical equipment installed in a cabinet and permanently connected to the mains supply without a plug and socket, with a permanent fixed earth connection. In that case, the typical leakage current in the range of a few to tens of milliamps is of no importance.

The effect of the higher frequencies can be difficult to predict. They can cause interference to other equipment if they are too high, which is the reason for fitting filter capacitors. They can also cause an RCD to operate. Simple RCDs do not have well-defined sensitivity to frequencies other than the mains frequency. Ideally, they would have a frequency response rather similar to that of the human body since they are intended to protect humans from electric shock. It happens that frequencies around 50/60 Hz are the most dangerous possible for humans.

Higher frequencies tend to flow in the surface of the body and not to affect the nervous system so much. If extreme they might cause skin burns but they are much less likely to cause fatal ventricular fibrillation. There are some superior RCDs sometimes referred to as “super-immune” which do have a carefully tailored frequency response. They are much less likely to give nuisance trips with VSD systems.

You should not under-estimate the effect of these higher frequencies. We have encountered installations where nuisance tripping of RCDs has occurred, where there was no filter. Adding filters cured the problem. It is counter-intuitive that connecting capacitors between the mains supply and earth could actually reduce the earth leakage current, but it can happen. The benefit in diverting the higher frequencies far exceeded the cost in terms of extra mains-frequency earth leakage.

Practical effects of VSD leakage current

The most common problem with leakage current is unwanted operation of an RCD.

In such a case, the first question is, whether an RCD is really needed? Sometimes RCDs are installed because of their perceived extra safety, and are not justified by the real situation. A permanently connected drive system in a dry environment is safe under all reasonably foreseeable fault conditions, and does not need an RCD.

If the RCD is installed because the earth loop impedance is too high to guarantee the operation of primary protection (e.g. with a TT earth system) then it is usually not necessary to use a 30 mA RCD. Very often a 1A RCD gives ample protection from earth faults, and is most unlikely to suffer spurious operation from a drive system.

For a small simple machine, which is more likely to be used with a pluggable power connection, usually, the motor is close to the drive. You can see from the previous explanation that then the filter capacitance could be reduced, giving sufficient filtering effect with less earth leakage. Control Techniques drives have a provision for disconnecting the internal filter capacitor to allow for this situation.

If radio noise emission must also be kept to a minimum, then for the smaller drives a special low-leakage variant of the external filter is available. You will understand now that this special filter has restrictions on the permitted length of the motor cable, which for small simple machines will not be a limiting factor.