Replacing a circuit breaker in MCB

If it is the old style sentry it fits on a fork rather than a prong

They cannot be replaced , without totally removing the bus bar

This is not recomended, unless you have appropriate test gear

to ensure it is not live.

The main switch off and the house lights off do not guarentee it is dead,

A multimeter is not a recognised way of proving dead either Afaik
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The power supply pack would need to be able to supply significant voltage as the current dropped off to test the MCBs ability to safely break the arc.
There are probably some things you'd have to take as read, and that would be one. For a manufacturer to test samples is one thing, but you'd actually want to avoid arcing in something you were going to put back.

"OK, Mr Jones - your MCBs were all fine - 100% success rate in testing. Of course they're trashed now, but don't worry, I've got new ones in the van." :D

That tester I linked to above only uses 2.3 - 20.3V....

You two are making this far too complicated - a spanner between line and earth works just fine. If you add a stop watch I am sure you could do a proper job - mind you, you will need to dress in the appropriate manner - these people can help :D
I took a lot at this today and it turns out to be quite simple really. Upon switching the main switch of the CU (RCD) to OFF, I was able to determine that only incoming power line remained live. Everything else was isolated (upper and lower busbar included). You need to remove a connection from the RCD to the lower busbar and then the lower busbar itself. Once you have done that, there is a spring clip underneath the MCB which you pull down with a screwdriver to release it from the rail. Still trying to source a replacement MCB but not completely necessary. The CU still does what it is supposed to done and self test works good.

Thanks to all for great feedback. As I read through the posts, I noticed the discussion around being able to test MCBs. Wouldn't a quick check be continuity? Otherwise regulated ampage test would be a lot more difficult and anyway what's to say all other MCBs have not degraded in terms of the ampage.
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As I read through the posts, I noticed the discussion around being able to test MCBs. Wouldn't a quick check be continuity?
That will tell you that the contacts are closed.

It won't give you a clue as to whether they will ever open, or at what current and in what time...

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