Shower tripping problem RCBO

[gazdkw82]

had a friend call me asking if I could help with his shower that keeps tripping.

On inspection it turned out the DP pull switch had broken. I swopped the switch and it seems ok. The power light on both the shower and the switch was showing power and this was confirmed with a meter.

however once the dial is turned on the shower to start drawing water the RCBO instantly trips.

It's a 39A shower and the rcbo is a 32A type B. I told him the rcbo is not large enough. He asked why it's only recently started tripping as it's been the same system for a few years. Tbh I didn't know the answer.

He did say he has had 3 switches changed in the past 2 years which I thought was strange.

Iv took some pictures and plan to change the rcbo to a 40A.

It's fed by 6mm t+e.

One thing that struck me was if the shower was tripping because of overload then why was it so instant. It's more like a short circuit. However if this is the case the short circuit is from the shower as the supply and load feed is good until the shower is physically turned on to draw water.

Any help would be appreciated

 

[Andy-1960]

I would be checking the heating element, sounds like it has failed to me

 

[gazdkw82]

How's best to check the heating element?

 

[Andy-1960]

Insulation test to earth

 

[davesparks]

To test a heating element measure the resistance of the element and the insulation resistance to earth.
The correct resistance for the element can be worked out from the power rating and voltage, if it's open circuit then obviously it's broken.
from your description it is most likely that the insulation test will reveal a fault to earth.

Are you sure it's 39A? Calculating backwards to get the power rating it appears that you've mixed up the power rating at 240V with the nominal voltage of 230V.
This would be incorrect because the power rating is different at 230V, most of these appliances will have the power rating at both voltages on their data plate these days.

 

[gazdkw82]

I initially used nominal voltage for my calculation (230) but then noticed the label listed 240 so I did the calculation for that.

It's a 9.5kw shower

 

[telectrix]

so your element resistance should be around 6.0 ohms. L-N. do that, then IR test it. (with it dissed from control board).

 

[davesparks]

It's a carefully designed label to fool the general,public. 8.7kW at 230V and 9.5kW at 240V

I think I see where you got the 39A from now, I assume you rounded down the result of 9500/240?

Regardless of this you are right that you wouldn't see a 32A MCB operate instantaneously with this shower, though it would damage the MCB over a period of time.

Before changing to a 40A RCBO make sure that the cable can handle the current the way it has been installed.
Bearing in mind that the shower is likely knackered and may need to be replaced they may have to fit a lower power shower.

 

[gazdkw82]

Just been and took another look. Resistance between the 2 element points was good. I just tested using a multimeter as I didn't have my multifunction tester so I couldn't do any insulation resistance tests.

I wasn't getting any resistance through the thermal cutout. However upon leaving iv realised I'm not sure if no resistance would mean no continuity?

Iv told him he needs to either replace the part of the shower entirely. He is desperate for the shower to be done so has opted for a replacement shower. I'm happy to fit tomorrow but just abit concerned that the problem will be something else.

However I just keep thinking about it and it has to be the shower that's the problem. When the power is turned on the light is on the shower and I confirmed there was voltage to the shower but as soon as you turn the dial to any setting (cold,warm,hot etc) it trips instantly.

It has to be the shower....surely?

 

[hightower]

It's 90% a L-E or N-E fault on the element. Should have had your MFT and you could have tested it for sure, but did you do a L-E and N-E continuity test with your multimeter? That would have still likely shown your fault.

 

[telectrix]

Just been and took another look. Resistance between the 2 element points was good. I just tested using a multimeter as I didn't have my multifunction tester so I couldn't do any insulation resistance tests.

I wasn't getting any resistance through the thermal cutout. However upon leaving iv realised I'm not sure if no resistance would mean no continuity?

Iv told him he needs to either replace the part of the shower entirely. He is desperate for the shower to be done so has opted for a replacement shower. I'm happy to fit tomorrow but just abit concerned that the problem will be something else.

However I just keep thinking about it and it has to be the shower that's the problem. When the power is turned on the light is on the shower and I confirmed there was voltage to the shower but as soon as you turn the dial to any setting (cold,warm,hot etc) it trips instantly.

It has to be the shower....surely?

what did the cut out read, close to zero or o/r?

 

[gazdkw82]

Nope. :-/

Feel abit of a idiot but you live and learn.

Next time I'll be more prepared

 

[gazdkw82]

Read 0.1

 

[gazdkw82]

Update.

Fitted new shower. Turned it on and guess what, same thing happened.

He had a spare 32A mcb in the board so for process of elimination I wired it into that. Works fine. Iv took the old rcbo away and will get a replacement tomorrow. Is there a way to test the breaker now it's out the board?

 

[telectrix]

you can only bench test the RCD function. assuming that the breaker will stay on when power is applied. ( unless you have a variable output 200A supply.