Residual Current Device (RCD): Working Principle, Applications

Compared to other energy sources, electricity has many advantages but also many risks. It is used on a daily basis by the general public and many accidents still occur, resulting in burns, fires and electrocution. International and national organizations have set up strict installation rules. Dependable protective devices have been designed by carefully analyzing the risks and consequences of equipment failures or incorrect use. Among these devices, RCDs (residual current devices) are recognized by international standardization organizations as an effective means to protect life and property.

What is a Residual Current Device (RCD)?

A residual current device (RCD) is a safety device that is designed to provide protection against electrocution and electrical fires by cutting off the flow of electricity automatically when it senses a ‘leakage’ of current from a circuit.

 

An RCD is used to disconnect a circuit whenever it detects that the electrical current is unbalanced between the phase conductor and the neutral conductor. 

How Does a Residual Current Device (RCD) Work?

A residual current device (RCD) works on the principle that in electrical circuits the incoming current is the same as the outgoing current. RCD incorporates a core balance transformer (CBT) having primary and secondary windings with a sensitive relay for instantaneous detection of fault signals.

The primary winding lies in series with the supply mains and load. The secondary winding is connected to a very sensitive relay. In faultless conditions, the magnetizing effects of current-carrying conductors cancel each other. There is no residual magnetic field that can induce a voltage in the secondary. During the flow of leakage current, an imbalance is created in the circuit which gives rise to leakage flux in the core. This leakage flux generates an electrical signal that is sensed by the relay and it trips the mechanism thereby disconnecting the supply.

The trip mechanism is operated at a residual current between 60-80% of its rated leakage current.

It is clear that the use of correctly selected RCD and good wiring practice can reduce the effects of electric shock and the possibility of fire risk significantly.

Electric shock may arise from direct contact with live parts, for example when a person touches a live conductor that has become exposed as a result of damage to the insulation of an electric cable. Alternatively, it may arise from indirect contact if, for example, a fault results in the exposed metalwork of an electrical appliance or even other metalwork such as a sink or plumbing system becoming live. In either case, there is a risk of an electric current flowing to earth through the body of any person who touches the live conductor or live metalwork.

Fuses and circuit breakers cannot provide protection against the residual currents flowing to the earth through the body as a result of direct contact. Residual current devices, provided they have been selected correctly, can afford this protection. They also provide protection against indirect contact under certain installation conditions where fuses and circuit breakers cannot achieve the desired effect.

Sensitivity of a Residual Current Device

RCD sensitivity is expressed as the rated residual operating current, noted I∆n. Preferred values have been defined by the IEC, thus making it possible to divide RCDs into three groups according to their I∆n value.

High sensitivity: 6 – 10 – 30 mA

Medium sensitivity: 0.1 – 0.3 – 0.5 – 1 A

Low sensitivity: 3 –10 – 30 A

RCDs for residential or similar applications are consistently high or medium-sensitive. High sensitivity is most often used for direct-contact protection (human protection, domestic installations), whereas medium sensitivity, particularly the 300 and 500 mA ratings, is indispensable for fire protection. The other sensitivities are used for other needs such as protection against indirect contacts (mandatory in the TT system) or protection of machines.

Residual Current Device Wiring Diagram

Diagrams of 2-pole and 4-pole residual current devices are shown below:

Classification

There are many different RCD types available, each being suitable for different equipment types.

Type AC

AC type RCD must be used for protection against AC earth leakage current.

Type A

A Type RCD must be used for protection against AC and pulsating DC (rectified AC) earth leakage current. It must be installed in any circuit where the main supply is likely to be rectified. Some examples of applications where this would apply are motor speed controllers (drives) and power tools.

Type B

B Type RCD must be used for alternating and/or pulsating current with DC components and continuous fault current. B Type RCDs are recommended for use with drives and inverters for supplying motors for pumps, lifts, textile machines, machine tools, etc since they recognize a continuous fault current with a low-level ripple.

Type F

F Type RCD is a special variant of type A with altered frequency characteristics, considering sensitivity to high frequencies. This type is only encountered since the introduction of IEC/EN 62423 but the solution is not completely new. The predecessor is type U, which was introduced to the market many years ago when the definition of properties of type F was not yet available.

Residual Current Device Applications

RCDs have wide usage areas. Some residual current device applications are:

1. Household and special environments

IΔn ≤30 mA, Standards make the use of these devices mandatory in all bathrooms, showers and private and public swimming pools and environments in which plugs and sockets may be installed without insulating or low safety voltage transformers.

2. Laboratories, service industry, and small industry

IΔn from 30 mA to 500 mA

3. Large service industry and industrial complex

IΔn from 500 mA to 1000 mA

Residual Current Device Internal Structure

Residual current device construction is shown in the picture:

FAQ about RCDs

Is an RCD the same as a circuit breaker?

A circuit breaker is a device designed to isolate a circuit during an overcurrent. A residual current device (RCD) is a device designed to protect against voltage leakage to the ground. For this reason, an RCD should always be used in conjunction with a CB to provide full protection from overload and leakage to the ground. 

Can an RCD be used as the main switch?

All RCDs have an isolation function. So RCD can be used as the main switch. You can use its operator handle for on/off operations. But remember that RCDs don’t have overload protection.

What should I do after RCB trips?

Switch off all the switches / MCBs connected in the circuit downstream with the RCD. Switch on RCD and simultaneously switch on the switches one by one. You will find during switching on of a particular appliance/switch RCD trips again and again. This shows that this is a faulty circuit/appliance. Rectify the fault and switch on the RCD.

What are the causes of unwanted tripping of RCDs?

Line side (upstream of the RCD)

• Loose connections.
• Mains borne disturbance.
• Site machinery/plant.
• Installed services.
• Lightning strike.

Load side (downstream side of the RCD)

• Wrongly specified RCD.
• Loose connections.
• Incorrect applications.
• Wet plaster/condensation.
• No discrimination between RCDs.
• Crossed neutral on split load board.
• N – E fault.
• High-standing earth leakage currents caused by householder / DIY faults. (e.g. nails/picture hooks)
• Moisture ingress. (appliances, sockets, etc)

Will an RCD trip on overload?

RCD doesn’t trip on overload. To protect circuits from overloads, an overload protection device must be used. (such as MCB, MCCB and overload relay)

What does 30ma RCD mean?

30mA is the sensitivity value of an RCD. It’s important because electrical shocks have serious effects on the human body >30mA. Check the table below:

How does the RCD test button work?

When the test button is pressed it allows the correct operation of the device to be verified by passing a small current through the test wire. This simulates a leakage to the ground by creating an imbalance in the current transformer (CT)

RCD rating plate