The Names

Possibly the greatest confusion with regard to RCDs concerns the plethora of names attaching to these devices. Various names are used in countries all over the world, but in Ireland and the UK the following are the most frequently used terms. RCD, RCCB, RCBO, ELCB. In the USA the terms GFI or GFCI are used.

I am going to start with the name ELCB because it is the name that causes most confusion.

ELCB = Earth Leakage Circuit Breaker.
There are two types of ELCB, the voltage operated device and the differential current operated device. For the convenience of this article only (and at the risk of causing even more confusion) I will refer to these as vELCB and iELCB respectively.

vELCBs were first introduced about sixty years ago and iELCBs were first introduced about forty years ago.

The principle of operation of the vELCB is as follows.
Under normal conditions the closed contacts of the vELCB feed the supply current to the load. The load is protected by a metal frame, such as in an electric cooker. The vELCB has a relay coil, one end of which is connected to the metal frame and one end connected directly to earth. A shock risk will arise if a breakdown in the insulation occurs in the load which causes the metal frame potential to rise to a voltage above earth. A resultant current will flow from the metalwork through the relay coil to earth and when the frame voltage reaches a dangerous level, e.g. 50 volts, the current flowing through the relay coil will be sufficient to activate the relay and thereby cause disconnection of the supply to the cooker and removal of the shock risk. It follows that the vELCB is essentially a voltage sensing device intended to detect dangerous touch voltages with respect to earth.

The level of shock protection provided by the vELCB was limited as these devices could not provide shock protection in the event of direct contact with a live part as shown in figure 1. vELCBs were also highly prone to nuisance tripping caused by momentary earth currents or currents originating in other circuits flowing through the relay coil.

The iELCB was introduced in the late 1950's, and operates on the principle shown in figure 1, and is today’s RCD. This device can provide protection in the case of direct contact, and in the case of indirect contact arising from a person touching a metal part with a dangerous touch voltage. The iELCB (RCD) is therefore essentially a current sensing device intended to detect current flow to earth.

For many years, the voltage operated ELCB and the current operated ELCB were both referred to as ELCBs because it was a simple name to remember and they both seemed to do the same job, although in different ways. However, the use of a common name for two different devices gave rise to considerable confusion in the electrical industry. If the wrong type was used on an installation, the level of protection given could be substantially less than that intended. To remove this confusion, IEC decided to apply the term Residual Current Device (RCD) to differential current operated ELCBs.

The RCD is now the preferred means of providing shock protection, and this term is most widely used internationally. Unfortunately, the RCD industry has had considerable difficulty in shaking off the old association with ELCBs, and many electrical contractors still ask for an ELCB when in fact they want an RCD. Hopefully this article will remove some of the confusion.

Continuing with the names:
RCD = Residual Current Device.
This is a generic term for the entire family of RCDs.

RCCB = Residual Current Circuit Breaker.
This is basically a mechanical switch with an RCD added to it. Its sole function is to provide protection against residual currents.

RCBO = Residual Current Breaker with Overcurrent Protection.
This is basically an overcurrent sensing circuit breaker (such as an MCB) with an RCD added to it. It has two functions,
a) to provide protection against residual currents and
b) to provide protection against overload currents.

The following terms are mainly used in North America. The principle of operation is exactly the same as for an RCD.

GFI = Ground Fault Interrupter.

GFCI = Ground Fault Circuit Interrupter.

Sorting RCDs by Products & Standards, and by Characteristics

Products & Standards
RCDs can be sorted by products and international product standards as follows.

• IEC61008 RCCBs (Type AC & Type A)
• IEC61009 RCBOs (Type AC & Type A)
• IEC62423 RCDs for DC fault current detection (Type B)
• IEC62640 SRCDs & FCURCDs (RCDs for socket outlets and fuse connection units)
• IEC61540 PRCDs – portable RCDs
• IEC62335 SPEs - RCDs for Electric Vehicles