mcb check

[millsy84]

can anyone tell me how you check a mcb will function under the designed short circuit capacity

 

[m4tty]

Do a PFC test

 

[rocker]

I think he/she means the functionality of an MCB.

Get something that will draw more amperage than the MCBs rating and plug it in?

 

[Hawk81]

I think he/she means the functionality of an MCB.

Get something that will draw more amperage than the MCBs rating and plug it in?

why do you say that? the op asks about short circuit, not overload. an overload of a circuit is not a short circuit.

 

[pushrod]

can anyone tell me how you check a mcb will function under the designed short circuit capacity

You do a Zs to check if it should operate in the event of an earth fault, but an R1+Rn is not even usually calculated let alone checked, however it would be expected to be a smaller value than R1+R2 so if that is ok you would expect it to function under short circuit.
MCB function as such is not checked in the same way as rcds are. You just rely on the manufacturer.

 

[martysparky]

That a thought, Is should enough...why are they not tested, what if a 20A didn't trip until 25 or not at all because the MCB was faulty and the cable fried?
Is this at all likely?

 

[pushrod]

That a thought, Is should enough...why are they not tested, what if a 20A didn't trip until 25 or not at all because the MCB was faulty and the cable fried?
Is this at all likely?

Well a brief 25A current is very unlikely to trip a 20A mcb anyway, because it has a fusing factor of about 1.45. This means that I2, the operating current of the device, will be in the region of 29A.

With BS 3036s the fusing factor has a higher value of 2 and this is why cables are derated by 0.725 at the design stage when being used with rewireables. They have a higher operating current which could potentially damage the cable if you didn't build in the necessary spare capacity.

Edit - i suppose the reason why mcbs aren't tested in real life is because it creates a dangerous situation. A little bit like if you have a car with a high ncap rating and super protective crumple zone for a head on impact you wouldn't drive your car into a wall just to test it

 

[rocker]

why do you say that? the op asks about short circuit, not overload. an overload of a circuit is not a short circuit.

Misread mate.

Fairly hostile.....

 

[Hawk81]

Misread mate.

Fairly hostile.....

I did not mean to be hostile, but as is happening alot on this forum I notice people are not answering the question but giving answers that are not relevent to the op.

 

[telectrix]

bit like knocking a nail thru a cable to test the RCD!

 

[martysparky]

But surely a way to test the function of an MCB would be to disconnect the cables of the circuit and use a tester connected to the MCB to measure the value and functionally test it. Otherwise it could well, for all we know, not trip until 31A, 40A, whatever, and surpass the current carrying capacity.

Obviously though, it seems, MCB funtionality must be trusted, where RCD's are not? and there faulty calibration deemed unlikely as its not a requirement. Until when I wonder.

 

[pushrod]

But surely a way to test the function of an MCB would be to disconnect the cables of the circuit and use a tester connected to the MCB to measure the value and functionally test it. Otherwise it could well, for all we know, not trip until 31A, 40A, whatever, and surpass the current carrying capacity.

Obviously though, it seems, MCB funtionality must be trusted, where RCD's are not? and there faulty calibration deemed unlikely as its not a requirement. Until when I wonder.

But if you want to test an mcb under short circuit conditions you would have to be generating a fault current of 100s or perhaps even 1000s of amp - don't think i'd want to be doing that test. Equally even if you are just testing its overload breaking you are generating quite a big current but not just for milli-seconds but for perhaps 200 or so seconds. Don't think my 8AA batteries on my tester will last very long.
When you are testing an rcd you are just producing a tiny fraction of an amp for milli-seconds so it is easier to do. Also , more importantly, it is the earth fault that could be electrocuting somebody an overload is not.

 

[telectrix]

true, earth faults electrocute, overloads cause fires, either way it's a one way trip to the crem.

 

[martysparky]

So, it could be argued that it would be an important but dangerous test. But we just rely on the manufacturer and installer with these components, but doubt the manufacturing/ installation capabilities of RCD's, Cable, etc etc.

I guess we can only test up to a point, the same with connections to fixed equipment.

I guess that reinforces the requirements of Part P. Wahhh

 

[telectrix]

we all put our faith in the BRB, why not trust manufacturers. wylex mcbs excluded!