MCB Tripping.. Help !!

[wallyanker]

Looking for some help please.

Friend of mine has just opened a new hair dressers near me and has had it all done out, re-plumbed, decor, etc. It was re-wired about 1 year ago. I get a call to say whenever she uses the showers, it trips the MCB after random lengths of time, but the RCD on that side of the board does not trip, so I go to investigate. My findings:

2 x brand new 8.5kw showers on seperate circuits 40amp Hager MCB 10mm2 T&E
Each cable run is approx 18-20 metres, all connections tightly secured, didnt have my meter with me so couldnt do any testing. Everything looks perfect to the eye, cable clipped surface mount through cellar and has no visible damage.

My question is, what can cause the MCB to trip without tripping the RCD?

Thanks
Pete

 

[telectrix]

generally an overload will cause this, or intermittent L-N fault. i have once had a fault where a damaged cable was intermittently shorting to E and that tripped the |MCB, not the RCD. but it's rare. you need to go back and test fully, clamp meter the current, IR test, including a RCD test. could be a dodgy MCB and / or RCD.

 

[Darkwood]

Agree with above if nothing shows up then swap over the 2 similar MCB's, it isn't common but occasionally mcb's are faulty but id also check the the terminations into 45 amp switch and shower as a loose connection is a likely suspect, and a common cause of tripping as it creates resistance and alot of heat and as the insulation of switch and cable gets hotter it will loose its resistive qualities and current will rise in line with temp'.

 

[Knobhead]

Are both MCB’s tripping or just one of them? If just one swap them over and see if the fault moves with it.

 

[telectrix]

and check that the busbar is correctly terminated in the MCB. sometimes it can be behind the "hole", which can cause no end of problems.

 

[Farmelectrics]

allways take your test equipment when attending any job big or small saves loads of time time = money lol

 

[Engineer54]

always take your test equipment when attending any job big or small saves loads of time time = money lol

Totally agree, totally pointless going to look at a fault with no test equipment!!!

 

[wallyanker]

Totally agree, totally pointless going to look at a fault with no test equipment!!!

Just called in as I was passing in the car to have a quick look, thought i might see something obvious, all test gear is in van.

Oh well, will call back in van and have a good nosey round with the probes !

 

[Deleted member 9648]

I,ve had instances of two heavily loaded MCB's side by side in the CU randomly tripping due to heat build up. If nothing else shows up and the two shower MCB's are side by side try moving them so they are not up against fully loaded mcb's.

 

[pushrod]

I have also come across intermittent tripping caused by really crappy terminations in a shower switch that were moving fractionally, as they warmed and expanded just enough to touch each other. Difficult to believe that someone could have walked away from such shoddy work!

 

[Guitarist]

First thing I would do after insulation testing the cable and showers and checking all terminations would be to clamp each of the supplies and see what they are actually pulling. Might be a good idea to leave the meter set on max and monitor it for a while to see if it spikes.

 

[Jay Sparks]

Just a thought, are the mcbs type B or C? Most mcbs/rcbos I have fit for a shower are type C. It may not be the problem but I just wanted to give you another angle to look from.

Jay

 

[Darkwood]

Just a thought, are the mcbs type B or C? Most mcbs/rcbos I have fit for a shower are type C. It may not be the problem but I just wanted to give you another angle to look from.

Jay

Showers are resistive loads and have no inrush to be taken into account so type B or C will not be a limiting factor here.

 

[Litttle Shonet]

or intermittent L-N fault

Just a quick one on this, please excuse my ignorance, but surely if there was a L - N fault, then any discrepency between the two would trip the RCD. Again please excuse me if I'm talking bulls**t

 

[D80]

An L-N fault won't affect the RCD at all, it will trip the MCB - as is the problem here.

 

[Darkwood]

Just a quick one on this, please excuse my ignorance, but surely if there was a L - N fault, then any discrepency between the two would trip the RCD. Again please excuse me if I'm talking bulls**t

L/N fault intermittent, short or low insulation is exactly that L-N, rcd look for earth leakage and as this isnt down to earth the rcd wont see it.
To note though in a L/N short circuit can sometimes trips the rcd but this is from my own assumption due to magnetic saturation of the device when the ampage can be in the 1000+ range for a split second until mcb trips and due to rcd looking at mA magnetic fields in normal use its plausable to me it will get saturated and activate.

 

[Guitarist]

Just a thought, are the mcbs type B or C? Most mcbs/rcbos I have fit for a shower are type C. It may not be the problem but I just wanted to give you another angle to look from.

Jay

As darkwood said, no inrush so type B for showers

 

[cakey]

Be nice to hear what was actually the problem?

 

[wallyanker]

Update.....

Been back over the weekend and can not find anything wrong with any terminations, connections, etc. IR test >1999Mohm. Split load board with 1 x 40a mcb on each side. Also tested RCD to make sure and all is good. So I turned to the shower unit itself....

Very very low water pressure at the shower head concerned me, especially when I raised the shower head above the unit and the water nearly stopped before going red hot and then the shower cut out (thermal cut out for element I think). MCB was still "on".

I told her to call the plumber back and do a pressure test at the shower end, then have the shower checked. It turns out that the pressure was indeed a little low but is not the main cause, the shower element is causing the MCB to trip so the plumber has replaced the shower and all is good so far.

Brand new gear causing all the problems, they dont make stuff like they used to!!!

 

[Darkwood]

Thx for update and as you said the thermal cutout was their to protect element but its continuous overheating and tripping will be the cause of the element failing and tripping mcb .... All down to the low water pressure, i suspect the shower lacks a pressure valve something missing on cheaper shower brands.