Life of an MCB

[Essexboy7]

Does anyone know how many times an MCB can trip before it is meant to be replaced?If there is any official publications on it can anyone point me to them?

Thanks in advance for any replies

 

[Marvo]

It would very much depend on the size of the fault currents. MCB's should be able to withstand large numbers of marginal overload trips but if it's a 3kA mcb for example and it's operated several times on a fault current that's > 2 kA then there would be an argument for replacing it.

 

[jay sean]

I think it is 3, have a look in the regs, I don't know where it is

 

[ayjay]

after 2 or 3 x theyre creamcrackered, imo

 

[Andrew Youll]

As Marvo has said depends in the fault current you would need to know the break capacity of your MCB and the PFC of the install to make an informed decision. 8ka fault current through a 6ka rated MCB is a change straight away for example as it is beyond the mcbs rated break capacity and will have most likely damaged the MCB. I have seen cases where the MCB hasn't operated but the HBC in the service head has blown as the PFC was beyond that of the MCB.

 

[Essexboy7]

cheers mate someone told me it is 2.I want to put it down as a reccomendation on a periodic but i want a point of reference when they ask me why

 

[scotsparky]

after 2 or 3 x theyre creamcrackered, imo

How do you come to that opinion?
for example on an older building say 30 years old how do you monitor how many times an MCB in its lifetime?

I would tend to agree with marvo. As for coding it in a Condition report i would put it as an observation and uncoded as there is no setfast number and there are a lot of variables.

 

[jay sean]

I think we were told at college (over 10 years ago)

 

[markc123]

Can't say i've ever seen that in the regs, if im wrong i appologise.

Clients exaggerate, every job you get to the powers tripped 900times, in reality its maybe 2. There's noway of policing how many times an MCB would/could/has trip(ped), so can't see why they'd be a concrete reg supporting the idea. You change it when you need to change it from a proffessional opinion...you can feel by operating the toggle whether its on its way out or not, or from the state of it by visually inspecting it.

 

[Marvo]

8ka fault current through a 6ka rated MCB is a change straight away for example as it is beyond the mcbs rated break capacity and will have most likely damaged the MCB. I have seen cases where the MCB hasn't operated but the HBC in the service head has blown as the PFC was beyond that of the MCB.

I would expect the MCB kA rating to be higher than the PFC if it's correctly specified.

As stated you can't police how many times it's tripped but if they're 6kA or less they're cheap and basically disposable items nowadays so we tend to err on the side of caution and if damage is even suspected we replace.

 

[Andrew Youll]

If correctly spec'd yes but have had managers who use the reg's allowing for a lower kA rating on an MCB if the suppliers over current protection exceeds the pfc to reduce number of board changes or MCB changes when carrying PIRs in the past. I agree its not ideal but is permitted in the regs don't have the bgb with me at the mo to get the reg number.

 

[scotsparky]

Marvo if it 6Ka or M6 which is the old BS3871 MCbs replacements can be expensive or impossible to get. Prime examples are
Ottermill unavailable anywhere exept sometimes ebay
MEM Alternatice MCbs made by Eaton but single cost about £80
square D Qoe range replacements are available from shnieder but expensice

 

[Essexboy7]

just checked the spec.It is a 63amp 3 pole breaker and says it is ok for 20,000 cycles mechanical operation and 10,000 cycles electrical operation


If it was only 2 or 3 it would need replacing after you have done 3 periodics

 

[SYKRAPS]

Would you apply the same thought process for both overload and short circuit currents for changing the mcb guys?

 

[Marvo]

Marvo if it 6Ka or M6 which is the old BS3871 MCbs replacements can be expensive or impossible to get. Prime examples are
Ottermill unavailable anywhere exept sometimes ebay
MEM Alternatice MCbs made by Eaton but single cost about £80
square D Qoe range replacements are available from shnieder but expensice

I forgot about this and you have a valid point with regard to the UK regs. Where I am we're allowed to replace any make MCB with any other make of MCB as long as it's correctly rated and identical form factor.

 

[Knobhead]

Anyone thought of looking on the ASTA web site?

 

[Engineer54]

I forgot about this and you have a valid point with regard to the UK regs. Where I am we're allowed to replace any make MCB with any other make of MCB as long as it's correctly rated and identical form factor.

Commonsense rules.... Well at least in S Africa!! lol!!

 

[Guest111]

trust Tony to come up with the obvious answer.

 

[Marvo]

Commonsense rules.... Well at least in S Africa!! lol!!

Yeah, on this particular occasion I'd agree. I can't help thinking the restrictiveness of the UK regs on this issue would overall be less safe by encouraging the use of second hand MCB's or the use of old and suspect units beyond the time they should.

 

[scotsparky]

Would you apply the same thought process for both overload and short circuit currents for changing the mcb guys?

An overload current is very small compared to a PFC or PSC so dont worry about it. It it can handle a 10,000A PFC then it will happily handle an overload up to 5 time rated value

 

[scotsparky]

I forgot about this and you have a valid point with regard to the UK regs. Where I am we're allowed to replace any make MCB with any other make of MCB as long as it's correctly rated and identical form factor.

marco BS3871,BsEN 60896 are interchangable and nothing wrong with using either BS 3871 has just been superceded but still valid and sometime second hand is fine i keep a load in my van.
The point i was trying to make was the cost of replacing after 2 or 3 faults.
In my opinion and experiance MCBs are failsafe and if they break will not allow a current to pass through it.

 

[SYKRAPS]

An overload current is very small compared to a PFC or PSC so dont worry about it. It it can handle a 10,000A PFC then it will happily handle an overload up to 5 time rated value

Just wondered what your reasoning was and obviously could not rely on the history that may be provided by a client as they don't know the difference in the majority of circumstances.

 

[scotsparky]

Anyone thought of looking on the ASTA web site?

American Society ofTravel Agents?? they got deals on?

 

[GLENNSPARK]

well as far as i know their rated to break a Max. fault of upto 16kA before firing...
the max. in the box on the side of them refers to the max. breaking capacity whilst still being good for continued service (6kA, 10kA etc)

 

[Engineer54]

marco BS3871,BsEN 60896 are interchangable and nothing wrong with using either BS 3871 has just been superceded but still valid and sometime second hand is fine i keep a load in my van.
The point i was trying to make was the cost of replacing after 2 or 3 faults.
In my opinion and experiance MCBs are failsafe and if they break will not allow a current to pass through it.

Not always so!! I've come across quite a few over the years, where the contacts have welded themselves together!! So they couldn't be switched off!! I will admit however, that most have been 3 phase jobbies!! lol!!

 

[Marvo]

marco BS3871,BsEN 60896 are interchangable and nothing wrong with using either BS 3871 has just been superceded but still valid and sometime second hand is fine i keep a load in my van.

To be honest I'm not familiar with the UK number designations or the in's and out's of the BS regs but I understood that your regs prohibited replacing a proteus MCB with a SquareD for example even if they're the same form factor. I heard several times on here that you must use the same make (manufacturer). Maybe I misunderstood in which case apologies in advance.

The point i was trying to make was the cost of replacing after 2 or 3 faults.
In my opinion and experiance MCBs are failsafe and if they break will not allow a current to pass through it.

Agreed if you exclude RCD's then overload protective devices are generally failsafe in nature. Once in a while we've experienced one that's overheated internally due to damage of the main contact surfaces but it's pretty rare.

 

[Marvo]

Not always so!! I've come across quite a few over the years, where the contacts have welded themselves together!! So they couldn't be switched off!! I will admit however, that most have been 3 phase jobbies!! lol!!

We've had mostly MCCB's with the symptoms, I can't remember a domestic type MCB that done this. Maybe we've just been lucky.

 

[Engineer54]

We've had mostly MCCB's with the symptoms, I can't remember a domestic type MCB that done this. Maybe we've just been lucky.

These weren't on domestic installations, mainly on commercial installs. There's not much different on basic MCCB's to that of standard MCB's. And yes in most of the cases, due to over heating of the internal contacts from loose cable terminal connections... But not all!!

 

[kingeri]

I went to a job a while ago where a ring final had been put on a 5a circuit breaker, the homeowner said it had been like that for many years and tripped out almost everyday day during that period. I inspected the breaker and it was in excellent condition and was still happily breaking overload currents.

 

[GLENNSPARK]

I went to a job a while ago where a ring final had been put on a 5a circuit breaker, the homeowner said it had been like that for many years and tripped out almost everyday day during that period. I inspected the breaker and it was in excellent condition and was still happily breaking overload currents.

and did you do a PFC on it?...this would have confirmed what it was actually breaking....

 

[Knobhead]

The two functions of a MCB are sustained (thermal) and instantaneous (magnetic) overload. With this instance it would have been switching (breaking) sustained overload not fault (instantaneous) current. It would be no more onerous that switching a kettle on and off.