Shower Trips RCD when turning off???

[ElectricalAsh]

Hi was called to look at a shower today which works perfectly but as you turn it off it trips the rcd in the C/U. I did an I/R test and loop test on the circuit and a ramp test on the rcd and the results where satisfactory so I informed the customer that the shower is faulty. Has anyone else had this problem? seems strange that the shower works fine though?

 

[ezzzekiel]

Leave shower runing - problem solved - next

Sounds like fault within shower unit and as ir and rcd tests show clear then not much more you can do except check internal connections for any obvious signs

 

[Deleted member 26818]

What's the IR when the shower is off?

 

[ElectricalAsh]

cheers for the replies. The ir was >299 between all conductors. They where taken with the shower disconnected. I did look for obvious faults but couldn't see any. thanks again

 

[Mr.Ian]

Shower switch worn ???? might be disconnecting the neutral but leaving the live connected for a split second when switching off???? would that create an inbalance on the RCD?? just like some microwave ovens, if the door is slammed they operate the short microswitch and blow their internal fuse as it's opening one switch before closing the other.

 

[ElectricalAsh]

Shower switch worn ???? might be disconnecting the neutral but leaving the live connected for a split second when switching off???? would that create an inbalance on the RCD?? just like some microwave ovens, if the door is slammed they operate the short microswitch and blow their internal fuse as it's opening one switch before closing the other.

thanks for the reply. Would this create an in balance in the RCD though as once the neutral was disconnected the circuit would be incomplete and it would not matter in if live was never disconnected (for the purpose of this) the RCD should not trip? also it trips if you use the pull cord or the shower itself.

 

[Mr.Ian]

The only other thing I can think is if it's got some form of mains filter/capacitor/suppression ??? didn't think showers did have as such but i've know faulty filters to to blow a plug top fuse on switching off on appliances. They can power up and run fine but blow when turned off.

 

[MarkieSparkie]

thanks for the reply. Would this create an in balance in the RCD though as once the neutral was disconnected the circuit would be incomplete and it would not matter in if live was never disconnected (for the purpose of this) the RCD should not trip? also it trips if you use the pull cord or the shower itself.

In an idea world your analysis might be valid. However in the real world, the situation is more complicated, due to the residual capacitances and leakage paths.
N-E residual capacitance exists in all installations, but the leakage through this capacitance is negligible because of the low potential (almost zero) between N-E. When the N pole of a double pole switch opens, the voltage across this capacitance will rise suddenly with a subsequent increase in N-E leakage current through this capacitance. This increase will be at a maximum if the switch N pole opens before the L pole. As the switch contacts open they will arc, causing HF voltage spikes, raising the N-E leakage even higher. If the switch has a slow break characteristic, often found in domestic switch gear, the effect will be even greater.
This phenomenon is most commonly witnessed in distribution boards with a separate front end RCD. Where opening the DB Main Switch causes a high voltage (near full mains potential) to charge the N-E capacitance of the whole installation, easily causing enough leakage current to flow to trip the RCD.

 

[malcolmsanford]

^^^^^^ nail head hit as usual with Mark