Q & A of the Day – Why is this RCD nuisance tripping when the lights turn on?

 

Question: I maintain an old rural railway station installation supplied via a single-phase TN-S supply from a very local pole-mounted transformer, which also supplies a few local cottages.

 

We have a 100mA S-Type RCD disconnector on the consumer unit, which supplies platform lighting via underground routed SWA cable final circuits to column mounted fittings. 

 

The trip is prone to nuisance tripping when the lights power up via a clock and photocell. This tripping also affects just one of the local cottages by causing its 30mA RCD trip to operate at the same time. 

 

We have carried out functional tests on our station supply and this confirms that we affect the cottage supply! 

 

Can you please advise on this scenario?

 

Answer: On this type of installation, unwanted tripping issues are normally related to filter networks on the lighting ballast causing initial earth currents to flow in order to charge the filter capacitors. Normally the time delay of the S-Type RCD is enough to prevent your unwanted trip, but for some reason your lights seem to be slow in charging - this may indicate a high resistance connection somewhere. 

 

Of course, you may also have an intermittent earth fault - water ingress in the cable? 

 

You state that the unwanted trips only occur when operating in 'automatic' mode from a timer, suggesting that you have an alternative manual supply mode which does not have the same issue - have you investigated the earthing regimes under both scenarios - are they different in some way? 

 

The local cottage must - in some way - be connected into the same earth path and be seeing the leakage current too. This suggests that there are some problems in the way your system earths are connected - there may be issues with circular current paths. 

 

One thing to look for would be daisy-chained earths between the station and the cottages - the solution would be to connect each individual earth point to the main earth directly. I hope this assist you in your problems.