Published: 10 June 2014
Category: Q&A
John asks this question, which concerns testing an outdoor lighting installation and the low impedance values found. It has been answered by SELECT (Theme – Testing, Tools & Equipment):
This Q & A is one of thousands posted in our Technical Expertise area, and answered on a daily basis by our Voltimum Experts.
Question: When testing an outdoor lighting installation, I find the impedance to be higher than that which the required isolation device allows. It is isolated using a 20A BS 88 fuse.
Does this mean that I must disconnect those lights there and then that are outside the required values?
Or can I report it and advise that an investigation be carried out?
Answer: In 559.10.3.3 for TN systems, a maximum five seconds disconnection time shall apply to all circuits feeding fixed equipment used in highway power supplies for compliance with Regulation 411.3.2.3 (TN system).
When checking the maximum earth fault loop impedance (Zs) from the tables using BS 7671 Table 41.4, you find are outwith the requirements of the table, you should advise the client / customer to rectify the fault as soon as possible to enable the circuit to disconnect in the correct time.
Also, it is a good idea to risk assess the area where the outdoor lighting installation is being used for any dangers you might envisage.
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