Characteristics

i) Based on Operation.
RCDs can be divided into two categories based on the means by which they detect and respond to residual currents. The two types are Voltage Independent (VI) and Voltage Dependent (VD). These are sometimes referred to as electromechanical and electronic types respectively. The VI type uses the output energy from the CT to activate a relay which in turn activates a tripping mechanism causing the RCD to trip. The VD type derives its operating energy from the mains supply and uses electronic circuitry to detect the earth fault current and to activate a tripping mechanism causing the RCD to trip.

ii) Based on Protection
The type of protection provided by RCDs results in additional categories as follows.

Type AC devices, which detect pure AC residual currents only.
Type A devices, which detect residual currents comprising of pure AC and pulsating DC.
Type B devices, which detect residual currents comprising of pure AC, rectified AC and pure DC.

RCDs are required to be marked with a symbol according to the above types, as follows.

III) Based on Response Time
RCDs are also divided into two categories based on their response time to a residual current, as follows.

General Type - having a response time < 300mS for (IΔΝ) and < 40mS for >5 (IΔΝ)
S Type - having a response time 150 - 300mS for (IΔΝ) and 40 - 130mS for (IΔΝ) .

As the name implies, General types are intended for general purpose use. However, S Types (Selective) are normally used in conjunction with a downstream General type RCD. The S type effectively provides discrimination in terms of the response time to residual currents. For example, when two RCDs are connected in series, the first RCD will usually be an S type and will trip only if the downstream RCD has failed to clear the fault.

Earlier, I explained that VI and VD RCDs are often referred to as electromechanical and electronic types respectively. However, there is an increasing use of electronic components in VI RCDs and the term "electronic" is becoming more of a misnomer.

Two factors have resulted in the increasing use of electronic circuitry in VI RCDs. These are:

i) Performance enhancement
ii) Nuisance tripping immunity

A basic VI RCD will be a General and AC type with limited sensitivity. By adding electronic circuitry, the device can be made to provide a delayed response (S Type) or improved sensitivity. Problems of nuisance tripping can arise due to voltage or current surges which can induce a current into the CT. By adding electronic circuitry, the resultant output from the CT can be snubbed or delayed so as to prevent an instantaneous response to the surge. (See Problems with RCDs, below).

iv) Based on Supply Conditions
RCDs can be divided into two categories which I will refer to as ML and EL.
ML - Does not trip on loss of mains supply
EL - Trips on loss or reduction of mains supply.

The EL type can be further divided into two subcategories as follows.
ELO = Opens on loss of supply and remains open on restoration of the supply.
ELR – Opens on loss of supply and recloses automatically on restoration of the supply.

Note: ML means “mechanically latched”. This may include latching by a permanent magnet.
ML Type RCDs will not trip on loss or reduction of supply whereas EL types will trip under such conditions.
EL means “electrically latched”, in that the EL type requires electrical energy to hold the contacts closed.
However, the ELR type will reclose automatically on restoration of supply. The ELO type should only be used where opening on loss of supply and remaining open on restoration of supply is required.
The terms ML, EL, ELO & ELR are not official terms. I have simply used them to better
describe RCD behaviour in response to supply conditions.

EL type RCDs are sometimes referred to as undervoltage release RCDs, and will trip automatically when the mains voltage falls below a specified level. This prevents voltage sensitive equipment from being operated at low voltages and avoids damage to such equipment. The exact voltage level at which the RCD trips will be specified by the manufacturer, but can sometimes be set in accordance with customer requirements.

In previous years, these devices tended to respond almost immediately to a low voltage condition, giving rise to problems of nuisance tripping. Following changes to RCD product standards, these devices will now generally not trip automatically unless the low voltage condition exceeds a specified period, typically 300 - 500mS. This ensures that the RCD will not trip in response to momentary dips in the mains supply.