When earth fault is too low…
We all should know that low voltage circuit breakers and fuses operate in the event of excess current arising from overload conditions and faults. As the statistics say, the most common fault type in LV installations is an earth fault, but… We’ve often seen the cases that the current flowing due to earth faults is too low to operate the overcurrent protection devices.
It’s important to note that the overcurrent protective device will not operate in the event of somebody making direct contact with a live conductor. Why? because the current which flows through the body to earth will be too low to operate the devices but will often be high enough to use fatal electric shocks.
These two problems can be obviated by the use of earth leakage protection devices.
The voltage-operated devices are no longer used but, for completeness, they consisted of a coil connected in series in the earthing conductor or between the metalwork of the installation and an auxiliary earth electrode. The device sensed a voltage rise in the metalwork with respect to earth and, when this occurred, tripped the circuit breaker.
The current-operated devices work on a dffferent principle, as illustrated in Figure 1 for a single-phase system. When the circuits are fault-free the current flowing in the phase conductor (Iph) will be the same as the current flowing in the neutral (In).
If there is an earth fault, some current (Ief) will flow back to the source via the earth path, creating an imbalance in the current flowing through the phase and neutral.
It is this imbalance that is measured, usually by passing the phase and neutral conductors through a core balance transformer. Any current imbalance produces a resultant magnetic flux which is picked up by the sensing coil and which, if it reaches a predetermined level, will cause the trip coil to operate.
The current imbalance needed to operate the device varies according to the application.
Most consumer units nowadays incorporate a split in the busbars, with an integral RCD providing earth leakage protection on circuits to socket outlets. The devices are not restricted to single-phase systems.
Figure 2 illustrates a three-phase RCD connected into the supply from a three-phase distribution board to a motor. In this particular case, the RCD may be set to operate at a leakage current of perhaps 500 mA since it is providing protection against indirect contact.
Note that the RCD is provided in addition to overcurrent protection devices such as miniature circuit breakers (MCB) and moulded case circuit breakers (MCCB). There are devices, known as residual current breaker with overcurrent device (RCBO) which combine the RCD and MCB functions.
Neither is the technique of earth leakage detection restricted to low voltage systems.
The technique is employed on high voltage systems although the are balance method is not the only one used. For example, another way to detect earth fault current is to monitor the amount of current that flows in the earthing conductor at the point of supply, using a current transformer. If the amount of current exceeds a particular value, a circuit breaker will operate to cut off the supply.
It is very important that the test button is used periodically to confirm the RCD’s serviceability because RCDs are sensitive devices and it is not uncommon for them to fail to danger; i.e. they fail in a way that means the contacts are closed but the device will not operate on demand.
This failure characteristic means that an RCD should not be relied on as the sole means of protecting against injury from direct contact. Another reason for this is that, for the RCD to operate in the event of direct contact, current of at least 30 mA must flow through the ‘victim’.
This amount of current is large enough to muse muscular contraction so, whereas it will almost is prevent electrical injury effects such as ventricular fibrillation in most cases, it may not prevent injury arising from the muscular contraction — such as falling off a ladder or being thrown against a wall.
Since the Electricity at Work Regulations aim to prevent injury, and since an RCD may not prevent an injury in the event of direct contact, its use as the sole means of protection against direct contact injury would be unlikely to satisfy the law. Having said that, the device’s value in providing supplementary protection against injury should not be underestimated.
There are some instances where the use of an RCD should be considered to be obligatory. These include:
- In socket outlet circuits in TT installations;
- In socket outlet circuits where it is foreseeable that the socket will be used to power outdoor equipment;
- In situations where there is an increased risk due, for example, to the presence of water. This would include the power supplies to power washers;
- Where 240 V hand tools and power tools are being used. Especially in work environments such as construction sites and workshops,
- In test areas where earth-referenced conductors may be exposed.
In these types of installations, a 30 mA RCD installed at the origin can be subject to nuisance tripping, so RCDs should be installed closer to the loads.
If RCDs are installed in series, discrimination between them can be achieved by building time delays into the RCDs, with the delay highest in those RCDs closest to the point of supply.
Source: Electrical safety and the Law by Ken Oldham Smith