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RCDs short for Residual Current Devices come in two types, electromechanical and electronic. The latter usually incorporated in other devices such as Portable RCDs, Combined RCDs and Socket Outlets, or RCBOs.

It is the smaller size which can be achieved which enables the electronic designs to be incorporated in the above units.

RCDs detect fault currents leaking to earth either through metalwork or unfortunately in some cases through a person's or animal's body.

Because of the way in which they work they can detect very small currents which cannot be achieved by any other device and provide the only practical method of protecting against the effects of these small currents.

Effects of Electric Shock
If a person comes into contact with metal which is carrying electric current and at the same time is earthed either through the other hand or the feet, a current will flow through the body dependant on the resistance of the body and the voltage of the supply. At 240 volts this current could vary between 240 thousandths of an amp. shortened to 240 milliamps and 80 milliamps.

For more details on the effect of ELECTRIC SHOCK click here

Firstly to have a low enough voltage that a dangerous current could not flow though the body. This would be 50 volts or below. Although this is used in special cases it is not practical for normal domestic or commercial premises.

Secondly to prevent people coming into contact with dangerous voltages. This is the object of barriers and enclosures. As we know from experience these are not foolproof.

The third and only practical way on mains voltage is the RCD which will detect that a fault current is leaking to earth and then cut off the supply rapidly enough to prevent dangerous effects.

Operation of the RCD

For details of how RCD's/RCCB's operate click here

Which RCD/RCCB ?
You will recall earlier that the level of current which flows through the body in the event of a shock situation is between 80 mA and 240 mA. It is essential that the detection of the RCD is below the minimum anticipated current through the body and in fact the recommended tripping current for shock protection is a maximum of 30 mA and this is the current recommended in the Wiring Regulations.

A tripping level of 100mA will a give degree of shock protection if it is not possible to use a 30 mA device.

A 300mA device should never be recommended for shock protection and is only intended for equipment and fire protection.

A 10 mA RCD should be used for 110 volt supplies because levels of less than 30mA could flow through a body in the event of a fault. A 10 mA device is sometimes recommended when the likelihood of shock is increased such as when people are working with live equipment

However at 10mA the likelihood of unwanted operation could increase at 240 volts so the 30 mA RCD should be used if possible.

The British Standard requirements for RCDs states that the RCD should operate between 50% and 100% of its rated tripping current. That is 15 mA and 30 mA for the 30 mA RCD. Most 30 mA RCDs operate at levels between 18 mA and 23 mA.

The Wiring Regulations recognize two ways to receive an electric shock. Firstly, indirect contact where a shock is received from metalwork made live by a fault and direct contact where a shock is received from metal intended to carry current such as a cable or electrical element.

In the case where a RCD is intended to provide indirect contact protection only, it is required to trip in less than 200 milliseconds at rated tripping current. In addition, for direct contact protection the RCD must trip in less than 40 milliseconds when 5 times rate tripping current is applied.

The only suitable instrument for testing a RCD is a RCD tester. A loop impedance tester cannot give an indication of the correct operation of a RCD and must never be used.

One RCD which does not comply with the requirements just discussed is the time delay RCCB. This device is time delayed or slugged so that it does not commence to operate for a predetermined time for instance 50 milliseconds which will allow downstream instantaneous RCDs to clear the supply without affecting the TD RCCB.

The time delay RCD is ideal in situations where the fastest tripping times are not required such as fire protection but must never be recommended for shock protection. The time delay RCCB must always be of lower sensitivity than the instantaneous RCD it is feeding in order to guarantee discrimination.

There are a number of queries which arise on RCDs.

Firstly the use of RCDs on reduced voltage such as 110 volts for a 240 volt RCD.

In the case of electronic RCDs they can only be used at their stated voltage because of the requirements of the electronic circuit.

As far as protection in the event of an earth fault is concerned on electromagnetic RCCBs there is no problem since the operation is dependant on residual current only. However the test circuit has to generate sufficient current to trip the RCCB and this is dependant on voltage.

Generally a 240 volt RCD will operate on the test button down to 110 volts. However according to BS4293 the test button is required to operate at 85% plus and minus 5% of the rated voltage. That is 192 volts for a 240 volt supply and 88 volts for a 110 volt supply. This latter operation cannot be guaranteed so although a 240 volt RCD will operate at 110 volts you cannot claim compliance with IEC1008 at that voltage.

RCDs are designed to operate at specific frequencies; in the case of the standard devices 50/60 Hz. They can be used at higher or lower frequencies but their tripping characteristics will change and they therefore cannot be recommended.

RCCBs to the latest standards are resistant to transient surges up to 250 amps over a period of 30 microseconds, and are marked accordingly.This makes them a lot more resistant to spikes created by lawnmowers and mixers etc.

RCDs above 30 amps rating are tested for an endurance of 2000 operations which is a mixture of manual switching, test button and out of balance.

RCDs have a fault breaking capacity of 3000 amps on their own., 10000 amps if used with a M10 MCB and 10000 amps if backed up with a BS88 fuse of 100 amps maximum rating.

This then is the basic technical information on RCDs.We will now look at applications and the types of problems which can occur once the RCDs are installed.



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Source - Contactum Distribution