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Sunday, 28 July 2013

ELCB's and Earth Leakage Protection, the stuff that keeps you safe.

 What is Earth Leakage Protection?

This is a topic about which volumes could be written but I will simplify as much as possible.
Electrical appliances are manufactured using many different substances. Some of these substances are good insulators while other substances are good conductors of electricity. Appliances that are metal clad can therefore be particularly dangerous as a potentially lethal voltage can be present on the metal, should a fault occur within the appliance.
For this reason, many types of Earth Leakage Protection devices were invented.

Types of Earth Leakage Protection devices.

Almost all of these devices available for the home today are of the ‘core balance’ type. Basically, a ‘toroid’ (wire wound around an iron ring) has the Active & Neutral conductors passing through the centre of it. If the current in one wire does not match the current in the other, a secondary current (the toroid is a currenttransformer & not a voltage transformer) is produced at the terminals of the toroid, which is then used to activate the trip mechanism.


Earth Leakage Protection devices are generally known as RCD’s (Residual Current Devices) or ELCB’s (Earth Leakage Circuit Breakers) & are available for 2 wire, 3 wire & 4 wire systems in a range of tripping currents & tripping speeds. The Domestic Electrical installation will either be a 2 wire or a 4 wire system.

Some models have adjustable trip currents (sensitivity) & are not recommended for home use.

It is an unfortunate fact of life that installations which have an adjustable ELCB invariably have it set to the highest available trip current, "to stop it tripping".


Most Earth Leakage Protection devices for home use are usually DIN mount style & will therefore plug into any modern distribution board. The installation should be performed by a qualified person as it should be tested at the time of installation. The testing requires ‘reference’ currents being induced into the device, which will test the trip times for each reference current. This testing can only be performed using specialist equipment & only by qualified persons.

Although one ELCB/RCD can be used to protect the entire electrical installation, this is not advised as nuisance tripping can result from ‘normal’ operation of some equipment. If three appliances are ‘leaky’ & you only have one ELCB/RCD protecting the complete installation, the sum of the leakage currents may be enough to trip the ELCB/RCD, whereas the individual leakage current from each ‘leaky’ appliance may not enough to trip the ELCB/RCD.

Instead, use an ELCB/RCD for each power circuit or install a 'Split Circuit' Consumer Unit (Distribution Panel), this will allow leaky devices to bypass the ELCB therefore retaining the maximum protection for the rest of the installation. See the Consumer Unit page for discussion of split circuit installations. 

Lighting circuits may also be protected but it is not recommended to protect more than half of these circuits. Trying to find your way out of a dark house in an emergency can be very dangerous. All ELCB/RCD devices should be of the same make & model (excluding overload trip current sizes in the case of the ELCB).


The ELCB/RCD can be turned off as required. Such operation will disconnect the circuit it protects from the electrical supply. Hence the ELCB/RCD is also a switching device.
As a general rule of thumb, it should be tested at least once a week (by the ‘press to test’ button), although more often is better.

  What to purchase.

          It is critical that the ELCB/RCD has a tripping current that does NOT exceed 30mA  (milliamps).

Its trip time must not exceed 20ms (milliseconds). Therefore, DO NOT PURCHASE ADJUSTABLE ELCB/RCD’S unless the adjustments cannot exceed these trip currents & trip times.

The ELCB/RCD MUST have a ‘press to test’ button.

Try to purchase a device that will not be affected by;
  • The presence of a DC component upon the normal supply (half wave DC). As an example, most hair dryers use half wave rectification on the ‘low heat’ setting. Some ELCB/RCD’s will not detect an earth fault under these conditions…a potentially deadly situation in a bathroom.
  • The presence of transients or harmonics upon the normal supply. An example of this would be computer power supplies, electronic sound amplifiers & some microwave ovens. These transients/harmonics can cause some ELCB/RCD’s to trip at unacceptably high leakage currents or not trip at all.
  • A broken Neutral conductor or a broken Active conductor (4 wire systems only).
There is normally a substantial price difference between the devices that are NOT affected by the above & the devices that ARE affected by the above.

What the ELCB/RCD cannot do.

It cannot protect against an Active & Neutral fault. For example, if you somehow become connected across the Active conductor & the Neutral conductor, the ELCB/RCD will NOT detect an earth fault & will not trip.
It also may not save the lives of people who are very old, ill or who have severe coronary problems.
Don’t forget!!! An ELCB/RCD can never replace a good earthing & MEN system. It is an electronic device that can fail at any time without notice. Test it regularly. If you can’t afford to properly earth your installation, start saving your pennies because earthing (& MEN) is the primary defense system against electric shock.


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