Why is Earth Leakage Protection Necessary in Low Voltage Installations?

When the ground fault is too low...

We should all know that low voltage circuit breakers and fuses operate in case of overcurrent caused by overload conditions and faults. As statistics say, the most common type of fault in LV installations is an earth fault, but we often see situations where the current flowing due to earth faults is too low to operate overcurrent protection devices.

Why is earth leakage protection necessary in LV installations?

It is important to note that the overcurrent protection device will not operate if someone comes into direct contact with a live conductor. So why? Because although the current flowing from the body to the ground is too low to operate the devices, it will often be high enough to cause fatal electric shocks.

These two problems can be solved by using earth leakage protection devices.

We should not forget that there are two general types of devices used for earth leakage detection: voltage-operated and current-operated.

Voltage operated devices are no longer used, but for completeness they consisted of a coil connected in series in the grounding conductor or between the metal structure of the installation and an auxiliary ground electrode. The device would detect a voltage increase in metal parts relative to ground and when this occurred it would trip the circuit breaker.

Current powered devices work on a different principle for a single phase system, as shown in Figure 1. When the circuits are fault-free, the current flowing in the phase conductor (lph) will be the same as the current flowing in the neutral (ln).

If there is a ground fault, some current will flow back to the source via the (left) ground path, creating an imbalance in current through phase and neutral.

Single phase RCD in simple circuit

This is unbalance, usually measured by passing the phase and neutral conductors through a core balance transformer. Any current imbalance generates a magnetic current picked up by the sensing coil which will cause the trip coil to operate if it reaches a predetermined level.

The current unbalance required to operate the device varies by application.

However, when the RCD is used to protect against electric shock, it must have a rated residual operating current not exceeding 30 mA when the residual operating current (in the current unbalance that makes the device work) and an operating time of 150 mA not exceeding 40 ms.

Most consumer units today include a baffle in the busbars with an integrated RCD that provides earth leakage protection to the socket outlets in the circuits. The devices are not limited to single-phase systems.

The image below shows a three-phase RCD connected to a supply from a three-phase distribution board to a motor. In this particular case the RCD can be set to operate at a leakage current of perhaps 500 mA as it provides protection against indirect contact.

Note that the RCD is provided in addition to the overcurrent protection devices such as miniature circuit breakers (MCB) and molded case circuit breakers (MCCB). There are devices known as residual current breakers with overcurrent devices (RCBO), which combine the functions of RCD and MCB.

Earth leakage detection technique is also not limited to low voltage systems.

three phase RCD

The technique is used in high voltage systems, but the balance method is not the only method used. For example, another way to detect the earth fault current is to monitor the amount of current flowing in the earth conductor at the supply point using a current transformer. If the amount of current exceeds a certain value, a circuit breaker operates to cut off the supply.

Each residual current circuit breaker in low voltage sources has a test button that, when pressed, creates an imbalance in the phase and neutral conductors passing through the transformer. While this no longer provides a test of the magnitude of the operating current or the tripping time, it does allow testing of the tripping mechanism. Proprietary test equipment is available for this purpose.

Because RCDs are sensitive devices and failure of RCDs is common, it is very important to use the test button periodically to verify RCD serviceability. That is, they fail, meaning that the contacts are closed but the device is not working when requested.

This fault feature means that an RCD should not be relied upon as the only way to protect against injury from direct contact. Another reason for this is that a current of at least 30 mA must flow through the 'victim' for the RCD to operate in direct contact.

This amount of current is large enough to induce muscle contraction, so while it almost prevents the effects of electrical injury, such as ventricular fibrillation, in most cases, it may not prevent injuries from muscle contraction, such as falling off a ladder or being thrown against a wall.

Since Electrical Regulations in the Workplace are intended to prevent injury and an RCD cannot prevent an injury in the event of direct contact, its use as the sole means of protection against direct contact injury is legal.

unlikely to meet. However, the value of the device in providing additional protection against injury should not be underestimated.

There are some situations where the use of an RCD should be considered mandatory. These are as follows:

In socket output circuits in TT installations;
In socket output circuits where the socket is intended to be used to power outdoor equipment;
For example, where there is an increased risk due to the presence of water. This includes the power supplies to the power washers;
Where 240 V hand tools and power tools are used. Especially in working environments such as construction sites and workshops,
In test areas where ground-referenced conductors may be exposed.
Many circuits and devices generate leakage currents to earth, for example through radio frequency installers. This means that in larger systems there can be quite a significant amount of earth leakage current flowing through the protective conductors under normal operating conditions.

In such installations, the 30 mA RCD installed at the starting point may be exposed to disturbing trips, therefore RCDs should be installed closer to the loads.

If RCDs are installed in series, separation between them can be achieved by creating time delays to the RCDs with the highest delay on the RCDs closest to the supply point.