Frequently Asked Questions about Electrical installations

HML Electrical Services offers the following information and data for your assistance only.  All electrical work on systems operating at over 50V should ONLY be carried by a qualified and experienced person – electricity can (and does) kill !

    * What is an RCD ?

    * Why 30mA ?

    * How fast does an RCD trip ?

    * What about the new 17th Edition Regualations ?

    * What are the new cable colours ?

    * My question isn’t answered here, what do I do next ?

  •       What is an RCD?

      An RCD (Residual Current Device) detects the difference in current flowing between the 2 live conductors (i.e. the fault current flowing to earth, possibly through you) and if this exceeds the specified rating, then it “trips” and cuts off the electrical supply to the circuit it is protecting.

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  •       Why 30mA?

      30mA is generally considered a “safe” current through the human body and, whilst you might feel it, it is unlikely to harm or kill you.

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  •       How fast does an RCD trip?

      A standard 30mA RCD of the type used in most domestic installations is designed to trip in under 200mS at its rated current and under 40mS at 150mA fault current. (There are also other types deliberately designed to operate slower than this.)

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  •       What about the new 17th Edition Regulations?

      For work carried out by HML Electrical Services due for completion before 1st July 2008, it is optional whether to use the “old” 16th Edition or the “new” 17th Edition as the Standard. After this date, all work must be carried out in accordance with the new 17th Edition. However, Testing and Inspection of existing installations after this date will not result in a “Fail” if they do not meet any new requirements.

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  •       What are the new cable colours?

      The new cable colours used in domestic lighting and power circuits are brown = phase (previously red), blue = neutral (previously black) and green/yellow = earth (previously green and officially known as cpc or “circuit protective conductor”).

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  •       My question isn’t answered here, what do I do next?

      Send your question by email and we’ll try to answer via this link :  

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