My electric shower is protected by an MK sentry unit with double pole RCD 30mA trip and MCB rated at 45 amps. The cable is 10mm Twin and Earth and the shower is rated at 10.5kW. Two weeks ago the MCB tripped. I reset it and it has performed ok until now. Today it tripped again and will not reset. Actually it will reset but trips as soon as the shower is switched on; i.e as soon as a load is applied. There are no signs of loose connections or overheating in the CU. and because the RCD has not tripped, I assume there is no earth leakage. Any ideas? A faulty MCB perhaps or is the problem more likely to be with the shower itself?
Shower MCB trips.
- Thread starter spongsdad
- Start date
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I take it you mean the double pole pull switch which isolates the shower unit. This appears OK Certainly it can be switched on and off without tripping the MCB . Perhaps I should have said that it is when I turn the shower on that the MCB trips.
Is there any kind of sound from the shower unit when the breaker trips out?
Its possible the heater can in the shower has broken down, do you have a multimeter? with the shower isolated (switch off the DP switch and the MCB to be sure), put the multimeter on continuity and see if there is a any kind of reading between the connecions into the heater can and the earthing point in the shower unit
Its possible the heater can in the shower has broken down, do you have a multimeter? with the shower isolated (switch off the DP switch and the MCB to be sure), put the multimeter on continuity and see if there is a any kind of reading between the connecions into the heater can and the earthing point in the shower unit
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I have now replaced the MCB and everything seems to be functioning normally. It looks as though the MCB has developed a fault. I have always placed great store by MK equipment and this is the first failure I have encountered. To those of you who are professional sparks, is MCB failure a common thing and can I rely on such a device to fail safe?
It happens from time to time, but normally in the way that after the MCB has cleared a fault that you can push the lever up, but it feels loose and the contacts inside stay open, or where its been exposed to sustained overloads, it'll weaken over time and might eventually trip when carrying a load less than In.
As to failing safe, well both the above failure modes fail safe, but as its a mechanical device there must be a risk (however small) that it'll jam, though with modern manufacturing methods its probably a small risk and this is backed up by the fact that there is no inservice test for them, the guidence is just to replace if they look worse for wear.
The risk is minimised by using recognised brands, rather than cheapo stuff that might be made in dveloping nations by companies who will fake technical standards approvals, though even a recognised brand is no guarretntee, sometimes even they can be subject to problems (think of the recent well known toy company inncident with lead based paints), and of course knock off products can enter the supply chain (cast your eye over them... if they look a tad suspect, then they might just be so!)
If a breaker does jam closed in a fault current situation though (quite unlikely), in a house, then the fuse provided by the electricity supply company should opperate before anything actually bursts into flame
As to failing safe, well both the above failure modes fail safe, but as its a mechanical device there must be a risk (however small) that it'll jam, though with modern manufacturing methods its probably a small risk and this is backed up by the fact that there is no inservice test for them, the guidence is just to replace if they look worse for wear.
The risk is minimised by using recognised brands, rather than cheapo stuff that might be made in dveloping nations by companies who will fake technical standards approvals, though even a recognised brand is no guarretntee, sometimes even they can be subject to problems (think of the recent well known toy company inncident with lead based paints), and of course knock off products can enter the supply chain (cast your eye over them... if they look a tad suspect, then they might just be so!)
If a breaker does jam closed in a fault current situation though (quite unlikely), in a house, then the fuse provided by the electricity supply company should opperate before anything actually bursts into flame
h'mmmmm
So 1mm² T/E will carry any current that will go through a 100A service fuse, will it, for the amount of time that the service fuse will pass it, without getting hot enough to start a fire?
You need to do the adiabatic equation to find out!
Probably not if the fault is at the far end of the circuit
, and impedances are high, currents lower and disconnection times longer and dispated I²t higher... I must admit I based my thinking on fault current of pretty high magnatude (a couple of ka), the numbers work then and nothing catches fire, unfortunatly in my haste to be clever I forgot to consider what would happen if the fault was a tad more than 2' from the DB Thanks for comments, gents. I'm still not certain if a device such as an MCB can fail in a "non failsafe" manner. If the fault had been on the shower,surely this would have produced a leakage to earth, tripping the RCD. Would not he MCB in these circumstances only provide overload protection, which is what I am relying on to avoid fire risk?
Incidentally, I am not sure where the 1.0mm T/E comes from. My circuit is using 10.0mm T/E
May I again enquire whether any of you have encountered failures with MK MCBs?
Incidentally, I am not sure where the 1.0mm T/E comes from. My circuit is using 10.0mm T/E
May I again enquire whether any of you have encountered failures with MK MCBs?
There is always a risk that a breaker will not fail safe, its a mechanical device afterall , but modern materials and methods serve to reduce this risk.
As to earth faults, a significant current to earth would probably trip the MCB (it'd beat the RCD so to speak), lower levels of current to earth 'earth leakage for example' for example caused for an extension lead dropped in a puddle would not operate the MCB and the RCD would operate.
The MCB provides overcurrent protection which can be caused by an
overload [too much current through an otherwise sound circuit]
or fault [high magnatude of current caused by a short circuit, or earth fault]
the 1mm came about simply because we were discussing how likely it is that a breaker thats failed closed [which itself is quite unlikely] would set fire to somehting in a fault situation
As to earth faults, a significant current to earth would probably trip the MCB (it'd beat the RCD so to speak), lower levels of current to earth 'earth leakage for example' for example caused for an extension lead dropped in a puddle would not operate the MCB and the RCD would operate.
The MCB provides overcurrent protection which can be caused by an
overload [too much current through an otherwise sound circuit]
or fault [high magnatude of current caused by a short circuit, or earth fault]
the 1mm came about simply because we were discussing how likely it is that a breaker thats failed closed [which itself is quite unlikely] would set fire to somehting in a fault situation
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