Interesting Electrical Snag (if your that way inclined)

[Matt2871]

Hi,

wondering if anyone could help with this little gem…… so……

i have just fitted an electric shower. i ran a 16mm twin core and earth from a dedicated consumer unit (an old electric heating consumer box now redundant after fitting oil central heating) through a 50 amp MCB to a 45 amp isolator switch to the shower unit……. when i was fitting the wiring i had the live and neutral lines disconnected from the consumer unit, but left the earth connected (as i couldn't see the problem with that)….. when i wired up the shower unit a green led on the shower suddenly caught my eye!! i panicked thinking i had done all the wiring live and ran to the consumer unit to check….. the live and the neutral line were disconnected as i had thought and only the earth was connected…… the led was very dim, but bright enough to clearly see….. thinking this was an earth leakage fault i disconnected one by one the all the earths from the main consumer unit to try and isolate the fault….. this didn't get rid of the dim led on the shower unit….. i then began isolating systems from the main consumer unit MCB's….. i found that with a certain lighting circuit isolated that the led turned off…… i found when disconnecting one of the lights on this circuit that it fixed the fault…….. i them wired up the live and neutral to the consumer unit only to find that when off the LED was glowing dimly again…… when i isolate the suspect lighting circuit it fixes the fault….. i disconnected every light fitting on the suspect circuit, but the LED still dimly glows when the shower is off


has anyone else seen this before….. or have any ideas of what i could check next??

Cheers…...

 

[PrenticeBoyofDerry]

Your problem is very likely to be a small induced current from the lighting circuit, are cables of each of these circuits routed in similar location?
Before you energise the circuit you should perform the required dead test on this circuit and before commission for service the required live tests must be performed. These test results must be compliant to those tabulated in BS7671.
As you have stated that you have run a new circuit, I do hope you are aware of the notification and certification requirements concerning installing new circuits in the UK. I also hope you are aware of the requirements for RCD protections on electrical equipment in bath/shower rooms and cable that of buried in walls, also the permitted routes that cables can be routed.

Could I ask you why 16mm2 cable has been used, did your calculation regarding design current, de-rating factors and voltage drop require this size cable?