Why do TT systems require RCDs to achieve 0.2s disconnection time?

by Steve on January 9, 2011

One of the main aims of Electrician’s Blog is to define and clarify anything relating to an electrician’s job and this obviously includes the BS7671 Wiring Regulations.
I normally refer technical questions to the forums or to an expert such as Chris Kitcher, but when Darren K emailed me and asked “Why RCDs are used for TT installations” I thought it would be an great subject for an article and the Q&A section.  RCDs are an important safety weapon in our armoury to prevent electric shock (fault protection formerly protection against indirect contact) and it is important to understand why they are used.

So armed with my Regs book and Chris’s Practical Guide to the 17th Edition I set to work.  I welcome your comments.

Why do TT systems require RCDs to achieve 0.2s disconnection time?

Types of fault
Faults in cables and equipment can occur between live conductors (L-L or L-N), or from live conductors to earth (L/N-E).  The Regs require that circuit breakers or fuses must disconnect the power supply to a circuit very fast should fault occur.  The fastest time being 0.2 seconds for TT systems.
Earth faults can be very dangerous as metallic parts of equipment and installations become live, at mains potential, and remain so until the circuit is disconnected.

TT Systems
TT systems require special attention because they normally have a higher impedance to earth via an electrode buried in the ground.  This high earth impedance would reduce an earth fault current and slow down the tripping of a breaker or fuse.

MCBs and faults to earth
TN systems generally have a very low earth impedance (which is good) and in most cases MCBs would meet requirements should an earth fault occur.
BS EN 60898 & BS EN 61009 MCBs, which are designed to trip within 0.1s, do meet the requirements for faults of low impedance between live conductors (L-L or L-N) in both TN and TT systems.  This is providing the earth impedance values of a circuit (Zs) comply with Table 41.3.  However where faults to earth (L/N-E) occur in TT systems, which have a higher earth impedance, they do not reliably meet the requirements for disconnection times.  This is why RCDs are used.

RCDs and faults to earth
RCD stands for Residual Current Device.  In the UK, they were formerly known as Earth Leakage Circuit Breakers (ELCB) which better describes their function.  RCDs simply monitor the Line and Neutral of a circuit and trip should an imbalance occur.  This imbalance would be caused by a flow of current from either Line or Neutral to Earth (L/N-E).  The most commonly used trip current rating of RCD is 30mA.  This a very low current and is ideal for earth fault protection in TT systems.

The On-Site Guide Section 3 states that
The required disconnection times for TT systems can (except in the most exceptional circumstances) only be achieved by protecting every circuit with an RCD.

Electrode Resistance in TT Systems
The Regs require that the earth resistance at the electrode (RA or Zs) be as low as practicable.  Table 45.1 refers to Reg 411.5.3 and specifies a maximum resistance required for a 30mA RCD to comply is 1,667 ohms, however Note 2 states that a value exceeding 200 ohms is not considered to be stable.  A 30mA earth current would trip the device within the required 0.2 seconds provided that conditions comply with 411.5.3 .

Fault Disconnection Times for TN and TT systems
The wiring regulations specify the requirements for Automatic Disconnection of Supply (ADS) by protective devices (fuses or circuit breakers) in instances of a fault.

As stated above, faults in cables and equipment can occur between live conductors (L-L or L-N), or from live conductors to earth (L/N-E).  The regs require different disconnection times for these faults depending on the type of earthing system for the installation which can be TN or TT.

Regulation 41.1  requires that:

  • circuits rated (32A or less) in TN installations must disconnect within 0.4 seconds (400ms)
  • circuits rated (over 32A)    in TN installations must disconnect within 5.0 seconds (5000ms)
  • circuits rated (32A or less) in TT installations must disconnect within 0.2 seconds (200ms)
  • circuits rated (over 32A)    in TT installations must disconnect within 1.0 second  (1000ms)

How is 0.2s Achieved?
A 0.2s disconnection time can be acieved by the use of RCDs.
An external Earth Impedance (Ze) below 200 Ohms is considered an acceptable and stable source of TT earth.
When a Ze of less than 200 Ohms exists then BS4293 RCDs are designed to trip within 0.2s but newer BS EN Types such as RCBOs may raise some concern as they are specified as having 0.3s disconnection times.  But that’s a great subject for another article!

Read more… –  Why use 0.3s RCDs for TT when they don’t comply with 0.2 disconnection?



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{ 6 comments… read them below or add one }

Steve November 8, 2011 at 16:18

Thanks for pointing out my error Lewis. This subject can be complicated enough without more stumbling blocks!

LewC November 6, 2011 at 20:12

V good explanation. Sorry to be picky but theres a typo on “circuits rated (32A or less) in TN installations must disconnect within 0.4 seconds (200ms)”. Surely 0.4 S is 400 mS. Just to clarify. Cheers

George Macdonald May 24, 2011 at 08:08

Please note – possible error on statement above on 5 seconds disconnection time for TT over 32A. Should read “TN”?
Thanks George- Now corrected.

electricians swansea March 5, 2011 at 13:25

thanks for the information, great article!

Andy February 7, 2011 at 18:51

I agree, good stuff. One of the more practical and easy to understand explinations I’ve seen.

Thanks, Andy

Ian Johnstone January 10, 2011 at 19:59

very interesting, good stuff!


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