Which replacement MCB to use.

[pawncob]

I have an MCB (6A) on the feed to the garage, which keeps tripping. There is nothing connected to the circuit in the garage and I can find no damage to the cable, which is underground.
The current MCB is a CH AM53 type 2 (BS6371) which is over 30 years old. It's in a block of six, but I assume they can be replaced individually.
Can anyone tell me what is the modern equivalent of this MCB?
TIA.

 

[Taylortwocities]

You must not mix MCBs. Just buy a replacement. Here you are

CH AM 68 6 TYPE 2 M6 SP MCB 6 AMP BS 3871 CIRCUIT BREAKER TRIP Same Day Dispatch - Circuit Breakers Electrical

CH AM 68 6 TYPE 2 M6 SP MCB 6 AMP BS 3871

circuitbreakerselectrical.co.uk

The maker is (or rather was) Cutler Hammer

But it’s usually not the MCB at fault. You probably have a fault somewhere. What testing have you done?

 

[pawncob]

I've removed the power point in the garage, leaving just bare wires. I've checked the cable physically at all accessible points.
When cable is disconnected from the MCB (Line only)multimeter shows 25ohm resistance at garage end.

 

[winston1]

I've removed the power point in the garage, leaving just bare wires. I've checked the cable physically at all accessible points.
When cable is disconnected from the MCB (Line only)multimeter shows 25ohm resistance at garage end.

25Ω between what and what?

 

[electrified]

if you have 25 ohms the mcb wouldn’t trip anyways it’s too high, what do you have 25 ohms between?

 

[opps]

You must not mix MCBs. Just buy a replacement. Here you are

CH AM 68 6 TYPE 2 M6 SP MCB 6 AMP BS 3871 CIRCUIT BREAKER TRIP Same Day Dispatch - Circuit Breakers Electrical

CH AM 68 6 TYPE 2 M6 SP MCB 6 AMP BS 3871

circuitbreakerselectrical.co.uk

The maker is (or rather was) Cutler Hammer

But it’s usually not the MCB at fault. You probably have a fault somewhere. What testing have you done?

As an aside... I once used this site for advice regarding a lighting MCB that would trip each time the lights were turned on in the morning. There was a 9 grid of switches in a kitchen extension. One switch would turn on all of the lights in one go. The advice was to fit a C class MCB.

Unlike previous "so called" electricians, I sourced a new but old stock MK MCB. Previous electricians had fitted whatever MCB they had in stock. The result was that the busbar is bent out of shape and and the MCBs tilt to the right slightly... oh and I also sourced a blanking plate to fill in the gap that they had left.

After that, I understand why different brands of MCBs should not be used in a CU.

 

[opps]

I have an MCB (6A) on the feed to the garage, which keeps tripping. There is nothing connected to the circuit in the garage and I can find no damage to the cable, which is underground.
The current MCB is a CH AM53 type 2 (BS6371) which is over 30 years old. It's in a block of six, but I assume they can be replaced individually.
Can anyone tell me what is the modern equivalent of this MCB?
TIA.

Do you mean that you have a CU in the garage? Or are you talking about the CU in the house.?

If in the house, I don't understand why anyone would run a 6amp supply to the garage.

If you mean that you have a 6 way CU in the garage, wouldn't the 6amp MCB be for lighting, and possibly, for external lighting, perhaps, along the garden (where water might be causing a short)?

I only ask because a customer recently had a CU that kept tripping in their summer house. They have lighting two way switching running from the end of the property to the summer house. I am not an electrician but eventually traced the problem to one of the five garden lighting circuits.

 

[pawncob]

It's 25 ohms between line and neutral in the garage, with line C.U. cable detached. It's a 35 foot run of 2.5 cable.

 

[flameport]

It's 25 ohms between line and neutral in the garage

Either there is a load connected, or there is a fault such as a damaged cable or junction box full of water etc.

With 230V applied, 25 ohms would result in a current of about 9 amps, therefore not surprising that a 6A MCB trips after a while.
If it is a fault of some kind, the resistance may change once it's connected to the supply which could result in the current being larger or smaller.

 

[pawncob]

Thanks for that. It's easier to replace the MCB first, rather than the cable, so I'll try that.

 

[pawncob]

Opps. The CU is in the house. The 6 amp supply is only for lighting in the garage.

 

[mikeyd]

If you measured 25 ohms with the cable detached from the CU, I would strongly advise you don't change the MCB. As said above, there is a load or fault downstream of the MCB.

 

[SUNRAY]

It's 25 ohms between line and neutral in the garage, with line C.U. cable detached. It's a 35 foot run of 2.5 cable.

Are you saying the 25Ω is in the garage CU with the feed cable disconnected?

25Ω at 230V should be a tad over 9A. Theoretically a 6A type 2 will never trip at 9A (but only just outside the curve) however the thing about faults such as this is the resistance may very well test as 25Ω but there's no telling what that resistance does when mains is applied to it. I had a fault fairly recently which tested as 'resistive short circuit' but tripping a MCB which it really should not have, but mine was way way higher than 25Ω and 'breaking down' under application of mains.


EDIT: Yet again started penning a reply then had my dinner before completing and posting. I see my words are moot now.

 

[ericmark]

I have talked many times about doing electrical work without test equipment, and I have to admit, my loop impedance tester, and my RCD tester my son now has, so I can't do all the testing I would like. However the basic testers I still have, low ohm tester with 200 mA test current, insulation tester using 250 and 500 volt DC, and an array of meters this is likely the main one, which can measure volts AC and DC and non contact, Amp AC and DC non contact, and ohms, the old one also measured frequency, also use the insulation tester and between the two I can test most things, and when required I borrow the meter from work.

It is so easy to measure what current is being used, and it seems likely in this case nothing wrong with the MCB, it is simply doing its job. But the meters cost money, the yellow one very old, before 1997 as bought in Hong Kong when still British, seem to remember about HK$ 250, the other two much newer, around £35 each, they are all cheap meters, as meter go, one can easy pay over £100 for them, the ones shown are likely the cheapest one can buy to measure down to 0.001 amp AC and DC so can test earth leakage.

Yes I could test mA with wires with cheaper meters, but not as easy, but it is no good guessing, measure, then you know for sure, wiring or MCB. Even the pros make mistakes, I remember connecting 25 x 110 volt 60 watt fluorescent lamps to a 16 amp MCB and it tripped, using the clamp-on I found using around 20 amp, so no wonder it tripped, changing the tapping from 110 to 127 on first 20 units cured problem. But without the meter I would not have realised what was causing it to trip.

 

[oldbutnotdead]

A multimeter isn't very useful for diagnosing your problem. If it is showing 25 ohms on what should be an open circuit you have either a fault or a mystery load.
Try measuring impedance L- E and N- E, see what you get.