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Home / Technical Articles / Inevitable in almost all electrical switchboards – Residual current device (RCD)

The magic of residual current device

Technology that RCD uses is so simple and so magical. The residual current device continually measures the difference between the value of the outgoing and incoming currents in the circuit it is protecting. If this difference is not equal to zero, that means there is a leakage current or a fault current.

Gladly seen in all electrical switchboards - Residual current device (RCD)
Gladly seen in all electrical switchboards - Residual current device (RCD)

When this current’s value reaches the residual current device’s set threshold, it automatically cuts off the circuit’s power supply. Simple as that.

The main function of residual current devices is to provide protection against indirect contacts. High sensitivity models also contribute to ensuring protection against direct contacts.

Let’s have the discussion about RCD operation, selecting tips, advantages and false tripping.

Table of contents:

  1. Composition of residual current devices
  2. Operating principle of RCD
    1. RCD operating in normal conditions, without fault
    2. RCD operating in condition with fault
  3. The choice of a residual current device
  4. The choice of protection device
  5. Advantages of residual current devices
  6. False tripping
    1. Causes
    2. Solutions

1. Composition of residual current devices

The residual current device consists principally of a core and a current-sensing relay.


1.1 Magnetic core

The magnetic core works like a transformer. The primary measures all the currents on the circuit being monitored, the secondary powers the currentsensing relay. If there is a leakage or fault current, the vectorial sum of the currents is not zero and results in a residual current.

Above the preset threshold IΔn, the current-sensing relay controls opening of the main contacts of the associated breaking device (switch or circuit breaker).


1.2 Current-sensing relay

The current-sensing relay consists of a magnetised coil which, as long as no fault current is present, holds an armature in the closed position. This armature is fixed on a shaft and is subject to force from a spring.

As long as the coil is not excited by a current, the permanent magnet provides an opposing force holding the armature in place which is greater than the force of the spring. If the coil is excited, the induced magnetic flux opposes the permanent magnetisation.

The force generated by the spring then causes the armature which controls the contact opening mechanism to move. Pretty simple, isn’t it?

RCBO phase + neutral
Figure 1 – RCBO phase + neutral

There are two kinds of current important for the work of RCD: leakage current and fault current.

  • Leakage current: Current that flows to earth when there is no fault in normal operating situations.
  • Fault current: Current that flows to earth via the exposed conductive parts or the protective conductor following an insulation fault.

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2. Operating principle of RCD

2.1 RCD operating in normal conditions, without fault

RCD operating in normal conditions, without fault
Figure 2 – RCD operating in normal conditions, without fault

I2 = – I1 
I1 + I2 = 0

The value of the outward current (phase) is the same as the value of the return current (neutral). If there is no residual current, no magnetic flux is created in the core. The current-sensing relay coil is not excited. The contacts remain closed.

The device works normally.

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2.2 RCD operating in condition with fault

RCD operating in condition with fault
Figure 3 – RCD operating in condition with fault

I2 ≠ I1
I1 +I2 = Id

The value of the outward current (phase) is different from the value of the return current (neutral). The residual current causes magnetic flux in the core, which generates a current that will excite the current-sensing relay.

For three-pole or four-pole residual current devices, all the conductors (phases and neutral) go into the core. But you should take it with caution:

The neutral conductor must always go through the residual current device and the PE conductor must never go through the residual current device.

For three - or four - pole residual current devices, all the conductors (phases and neutral) go into the core
Figure 4 – For three – or four – pole residual current devices, all the conductors (phases and neutral) go into the core

2.2.1 At the first fault, in a TT configuration

Placed at the front end of an installation, the residual current device will detect fault currents as soon as they arise. It also means that hard-to-obtain earth connection values need not be demanded.

Placed on each outgoing feeder or each group of circuits, it means that protection can be discriminating if the exposed conductive parts are not interconnected.


2.2.2 At the first fault, in a TN configuration

Placed on each outgoing feeder, it guarantees the tripping conditions in the event of excessive line lengths and poorly controlled usage.

Placed at the head of a group of circuits, it provides protection when the exposed conductive parts are not interconnected (separate buildings, remote use).


2.2.3 At the second fault, in an IT configuration

Placed on outgoing feeders whose conditions do not guarantee protection (line lengths often limited in it by a lower fault current than in tn), it guarantees disconnection.

Placed at the head of a group of circuits, it provides protection when the exposed conductive parts are not interconnected (separate buildings, remote use).

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3. The choice of a residual current device

The choice of a residual current device depends on the required level of protection (trip threshold sensitivity I∆n), on the nature of the associated breaking device (circuit breaker or switch) and the specific conditions of use (delayed, discriminating, immune).

Dual RCD arrangement
Figure 5 – Dual RCD arrangement (100 mA RCD used for fault protection and 30 mA RCD used for additional protection); photo credit: Voltimum

3.1 Determining the trip treshold

there are three families of residual current devices, referred to as high, medium and low sensitivity.


3.1.1 High sensitivity: I∆n < 30 mA

These are used on power socket circuits, in damp rooms, mobile installations (building sites, fairs, etc.), agricultural buildings or when the earthing conditions are defective.


3.1.2 Medium sensitivity 30 mA < I∆n < 500 mA

These are used on the circuits of fixed installations (principally on TT systems). they provide discrimination with high sensitivity devices. 

They ensure protection under minimum short circuit conditions (lengths of lines in tn and it systems) and limit the fault currents (risk of fire).

3.1.3 Low sensitivity: I∆n > 0.5 A

Used in TN and IT systems, these provide discrimination with high and medium sensitivity devices.

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4. The choice of protection device

4.1 RCD without overcurrent protection (RCCB)

Conforming to standard IEC 61008, this breaks the circuit, but does not provide protection against overcurrents. A protection device, such as a circuit breaker or fuse, must therefore be used with it, which will also protect the RCBO.


4.2 RCD with overcurrent protection (RCBO)

Conforming to standard IEC 61009-1, this both breaks the circuit and protects against overcurrents (short-circuits and overloads).

It is available in several forms:

  • Modular monobloc
  • Add-on module for modular device
  • Residual current relay with separate core used with a circuit-breaker

4.3 RCCB upstream of the overcurrent protection devices

The part of installations between the RCCB upstream and the protective devices downstream must be subject to precautions that are designed to reduce the risks of indirect contact (wiring in ducting, attached wires, busbars).

RCCB upstream of the overcurrent protection devices
Figure 6 – RCCB upstream of the overcurrent protection devices

Residual current devices are equipped with a “TEST” button. This simulates a fault current. A test must be performed once a month.

RCD test button
Figure 7 – RCD test button

Residual current trip thresholds are usually guaranteed for a low temperature down to – 5°C. There are also some special RCD versions othat can hold on – 25°C.

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5. Advantages of residual current devices

The total assurance of the protection provided by neutral earthing systems depends both on the design (calculation) rules, construction (lengths of lines, quality of the earths) and above all on upgrades to and use made of the installation (extensions, mobile loads).

In the face of this uncertainty, and the eventual risk of a reduction in the level of safety, the use of residual current devices represents a “solution” that can be used in addition to earth connection schemes.

Medium sensitivity (300 or 500 mA)

This avoids the rise in the energy of fault currents that may cause a fire (protection of property).


High sensitivity (30 mA)

This maintains protection against indirect contact if the earth is poor or the protective conductor is broken. It is used in addition to protection against direct phase/earth contact (protection of people).

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6. False tripping

6.1 Causes

6.1.1 Leakage currents

LV electrical installations have permanent leakage currents, which are not due to faults, but to the actual characteristics of the insulation of the devices and the conductors. They are generally a few milliamperes in an installation in a good state of repair and do not cause any unwanted tripping.

The development of receivers integrating increasing amounts of electronics with associated switching mode power supplies and filtering is leading to higher leakage currents. A single computer terminal incorporating several devices (CPU, screen, printer, scanner, etc.) may have a leakage current of a few milliamperes.

Supplying several terminals from the same socket or the same circuit can therefore quickly lead to a total leakage current that causes high sensitivity residual current devices to trip.


6.1.2 Transient currents

The capacitive effects of the installation, switching overvoltages on inductive circuits, electrostatic discharges or lightning strikes are all momentary phenomena which are not faults in the real sense, and against which the residual current devices must be rendered immune.


6.1.3 Presence of DC components

DC components may circulate following the failure of certain electronic power supplies. These may alter or even destroy the operation of residual current devices if they are not protected accordingly.


6.2 Solutions

6.2.1 High leakage currents

  • Divide the circuits and protect them separately so as to limit the number of devices for each of them, ensuring there is vertical discrimination.
  • Use class II devices when they are available – supply devices with a high risk of leakage via a separation transformer.

6.2.2 Transient currents

  • Limit these by ensuring good equipotentiality of the installation.
  • Use cables with a protective conductor connected to earth, even if this conductor is not used (supply of class II equipment).
    Cables without a protective conductor can generate transients by capacitive effect.
  • Use delayed residual current devices (type S), which allow transient currents to pass during the delay phase.

Go back to Content Table ↑

Source: Electrical hazards and protecting persons by Legrand

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

6 Comments


  1. MingChing Teng
    Feb 25, 2020

    Dear Mr. Csanyi,

    Through reading your article” Underground Residential Distribution Layouts”, some questions popped up. It would be highly appreciated if you would be so kind to answer the following questions:
    1. Re the detailed construction of the three twisted conductor bundles of Figure 1, are the constituent conductors of every
    bundle enamelled, or, without any insulation, just twisted and compressed?
    2. Please send the. catatog for the rcds, that can be used to detect and protect the cables, against the leakage currents at
    microamp level.

    Sincerely,

    MingChing Teng
    Ray Technology, Inc.
    Taipei, Taiwan
    Feb. 25, 2020


  2. Stephen Smyth
    Feb 13, 2020

    Wow such a good techniques, ready to learn more about this.


  3. Fazal Muhammad Khair
    Feb 13, 2020

    Very useful for me.


  4. Robertson
    Feb 13, 2020

    Good techniques


  5. David
    Feb 12, 2020

    Would you still recommend rcd protection on a metal clad, district heating plant room that is on a TT earth and is critical for continuous operation (i.e. The plant needs to be operational continuously with no nuisance tripping). It is located in a public area.


  6. Rajkumar bhardwaj
    Feb 12, 2020

    Which type of rccb is recommended in 4kv house hold protection specially protection from bathing under running gezure

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