MCB trips on shower when toilet gets flushed

[mancmicky]

This is a real head scratcher for me - I've recently had to replace an electric shower (with a B&Q cheapy special - element went on the old one), the new one works fine (8.5kw, same as old) & I've test run at maximum heat for 15 mins (the minimum time my teenage daughters consider it requires to get clean) with no problems.

When in real use I usually get screams of abuse as the MCB (40amp) trips resulting in freezing cold water on said daughters (not all bad news then... )

Spent today investigating & trying to duplicate the problem, all wiring & connections ok until the downstairs loo got flushed & the MCB tripped.

Obviously (?) there is a water pressure / scald sensor that gets brought into play but why would this trip the MCB?, water pressure is usually ok round here (2 bar) & the stop cock if fully open

Help & advice please - If I need to I can replace with, say, a 7.5kw which might not trip or just get a more "quality" 8.5kw one but don't want the hassle if the replacement is likely to do the same.

 

[breezer]

since flushing the loo reduces water flow the shower must be geeting too hot, drawing more current, hence mcb trips.

i would be inclined to check water connections first

 

[plugwash]

i'd expect overheating to reduce the current draw (at least until the point of permanent failure was reached) not increase it.

 

[mancmicky]

Thanks for taking the time to reply - Yup, water flow was my thought as well, but why would reduced flow result in more current draw? - I can understand reduced flow resulting in hotter water & possible safety cut out but my understanding (& quite, quite willing to be corrected) is that the 8.5kw would have a maximum draw of approx 36-38amps which is below the 40amp MCB.

Or am I mis-understanding how an electric shower works?

 

[ricicle]

since flushing the loo reduces water flow the shower must be geeting too hot, drawing more current, hence mcb trips.

How does an decrease in water flow cause the current in an electric shower to increase (apart from the milli-amps to light the warning neon)?

 

[breezer]

the heat is not being taken away fast enough by the water flow, so the element gets hotter.

 

[davy_owen_88]

But as the element gets hotter it's resistance will rise and the current will actually drop.

 

[breezer]

yes, iwill give you that one, but the question is how hot is hotter?

and what else could it be? if i am wrong?

 

[bernardgreen]

IF it is the over heat sensor reacting to reduced water flow then the logical fault would be that the wiring to the warning lamp has a short circuit across it. When the sensor operates and puts live to the lamp the short circuit means a live and neutral short.

 

[ColJack]

or failing that, you've got a leaking pipe at the back of the toilet and a JB under the floorboards just under it...

does the trip re-set properly and easily when you have to?

 

[mancmicky]

Yup, resetting the MCB works easy & fine with no problem until the next time the shower runs / loo flushes. All wiring is external to the bathroom - just the shower cable from the DP switch goes directly through the wall into the back of the shower unit. I've a split load CU with the shower & power on the protected side, it doesn't trip the main breaker which I would expect if there was a fault. Ta for all the suggestions so far.

 

[tim west]

Are you sure it's an MCB or could it be an RCBO?

 

[mancmicky]

Replaced with 7.4kw, works fine, weird!. Thanks for all the help

 

[2scoops0406]

or failing that, you've got a leaking pipe at the back of the toilet and a JB under the floorboards just under it...

does the trip re-set properly and easily when you have to?

Now I doubt that the OP flushed the bog in the same room as his showering daughter