TT Systems

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

9 January 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 [...]

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

9 November 2010

How can 0.3s RCDs comply for TT systems which require 0.2s disconnection? 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 BS7671 Wiring Regulations require that breakers or fuses must disconnect a circuit within a specific time in the instance of a fault. 0.2s [...]

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TT circuits require a disconnection time of 0.2 seconds

22 May 2009

  TT circuits require a disconnection time of 0.2 seconds.  (BS7671 Table 41.1). Q How can this be achieved when the Max Loop Imp Tables in BS7671 only deal with 0.4 and 5 seconds?   A BS7671:2008 The note below Table 41.1 states that: TT disconnection time of 0.2 secs up to 32A & 1 [...]

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TT Systems. Zs reading higher than tabulated

22 March 2009

Q I am measuring a Zs of 140Ω on my final circuits. Table 41.3 gives a maximum of 1.44Ω. I know that a Ze below 200Ω on a TT system is acceptable (Table 41.5 note 2). Is my Zs ok? A Yes it is acceptable. As long as your circuit is protected by an RCD. [...]

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TT systems. Bonding in bathrooms

22 March 2009

Q Does the dispensation in Regulation 701.415.2 to omit supplementary bonding in a bathroom apply to TT systems?   A Yes it does apply to TT systems. Regulation 701.415.2 states that supplementary bonding may be omitted where: i) All final circuits in that location comply with 0.4 sec disconnection ii) All final circuits in that [...]

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TT Systems. Time-delay RCD main switch

22 March 2009

Q Is a time-delayed RCD main switch required in the consumer unit of a new domestic installation supplied by a TT system?   A Provided that the consumer unit is of all insulated construction and all the final circuits are RCD-protected, a time-delayed RCD main switch is not required. How is 0.2s Achieved? A 0.2s [...]

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