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Which type of residual current device (RCD) to use and when?
Which type of residual current device (RCD) to use and when?

Total earth loop impedance

We know how (very) important the total earth loop impedance Zs is in the reduction of shock risk.

However, in TT systems where the mass of earth is part of the fault path, the maximum values of Zs given in the IEE Regulations may be hard to satisfy. Added to this, climatic conditions will alter the resistance of the earth in such a way that Ze may be satisfactory in wet weather, but not in very dry.

Operating principle of the residual current device
Figure 1 – Operating principle of the residual current device

The regulations recommend, therefore, that the preferred method of earth fault protection for installations in TT systems be achieved by RCDs, such that the product of its residual operating current and the loop impedance will not exceed a figure of 50 V.

Residual current breakers (RCBs), residual current circuit breakers (RCCBs) and RCDs are one and the same thing. Read more about this.

Modern developments in CB (circuit breaker), RCD and consumer unit design now make it easy to protect any individual circuit with a combined CB/RCD (RCBO), making the use of split-load boards unnecessary.

In domestic premises the use of 30 mA RCDs is required for the protection of all socket outlets rated at not more than 20 A, for all circuits in a bath or shower room and for cables embedded in walls and partitions at a depth less than 50 mm. Socket outlets not intended for general use, for example those provided for non-portable equipment such as freezers, etc., are exempt from this requirement, provided they are suitably labelled or identified.


Applications of RCDs

30 mA

Applications
All socket outlets rated at not more than 20 A and for unsupervised general use.
Mobile equipment rated at not more than 32 A for use outdoors.
All circuits in a bath/shower room.
Preferred for all circuits in a TT system.
All cables installed less than 50 mm from the surface of a wall or partition (in the safe zones) if the installation is unsupervised, and also at any depth if the construction of the wall or partition includes metallic parts.
In zones 0, 1 and 2 of swimming pool locations.
All circuits in a location containing saunas, etc.
Socket outlet final circuits not exceeding 32 A in agricultural locations.
Circuits supplying Class II equipment in restrictive conductive locations.
Each socket outlet in caravan parks and marinas and final circuit for houseboats.
All socket-outlet circuits rated not more than 32 A for show stands, etc.
All socket-outlet circuits rated not more than 32 A for construction sites (where reduced low voltage, etc. is not used).
All socket outlets supplying equipment outside mobile or transportable units.
All circuits in caravans.
All circuits in circuses, etc.
A circuit supplying Class II heating equipment for floor and ceiling heating systems

100 mA

Applications
Socket outlets of rating exceeding 32 A in agricultural locations.
Where Loop Impedances are too high, RCD ratings can be calculated.

300 mA

Applications
At the origin of a temporary supply to circuses, etc.
Where there is a risk of fire due to storage of combustible materials.
All circuits (except socket outlets) in agricultural locations

500 mA

Applications
Any circuit supplying one or more socket outlets of rating exceeding 32 A, on a construction site

RCD installed on a DIN rail //

When and which Residual Current Device to use?
When and which Residual Current Device to use?

Reference: Electric Wiring: Domestic – Brian Scaddan IEng, MIET

About Author //

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

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