Modern MK RCBOs in old (1986 vintage) Sentry Consumer Unit

16 Apr 2010
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United Kingdom
I have an old MK Sentry Consumer unit fitted with LN59xx MCBs:


I want to replace the LN5905 5A MCBs with H6832s (6A, 30mA, Type A) RCBOs so that the lighting circuits are RCD protected (given that I cannot find an RCD to fit this CU).

I can see that the busbar isn't compatible, so I will also need a new busbar (H5511s) and couple of modern MCBs H5945s and H5932s to replace the old MCBs for garage power and the PV feed.

My question is what part do I need to connect that 25mm tail to the the new style busbar - the MK Sentry catalogue is awful quiet about parts to connect the live input to the busbar? Or does the tail just insert into the bottom of one of the MCB/RCBOs along with the finger on the busbar?

Thanks, David
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Might be easier just to replace the whole consumer unit. Are those MCBs mounted on a standard DIN rail it's hard to tell. Have you got a means of isolating that Board?

Is there enough room for the top of the RCBO's to fit in?
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Except it's a double CU so prefer not to replace, you are only seeing one of two rows. Yes they are mounted on a standard DIN rail. As for isolation yes I have an external 100A isolator before the CUs (yes there's another one as well as this double row one).
Might be easier/better to replace all the MCBs with newer type& appropriate Bus Bar. The "tail" should fit into the Cooker MCB along with the Bus bar
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All you can really do is buy the stuff you think will fit, and try it and see.

Potential waste of money if it doesn't fit.

Also, you would be fitting parts that are just not designed to go with that board, which is likely to be a no-no.

The board is likely to have some plastic forming the enclosure. Although no requirement to replace such things, it would make sense to change the whole thing to be up to date.

I realise you don't want to change it.

Send some more pics of the whole thing.
I have tried in the past to use old boxes, where the problem lies is the hole the MCB/RCBO sticks through, normally better to replace whole board.
Excellent point - the RCBOs are bound to end up at the wrong height, and the cover won't go on properly.
Current range are not compatable with that board, one further issue you might came up with if you get it to fit, is that the height of the rcbos is probably too high to be able to dress teh cables in without struggling.

If you are determined to replace components in the existing unit, then you might as well not worry about it being MK, as you'd not end up with a type tested assembly anyway, so you may as well get some compact RCBOs and busbar thats compatable with them, You'd not feed onto the bottom of the busbar like that, you could either fit a 100A DP isolator from the band you are using or some makes offer a direct connection block which would only cost you 1 module of space.

Don't generally like to see mix and match, but if the breakers, busbar and incomming device are all being changed, leaving only the enclosure (and the separate section below) as MK, then I don't see it being such an issue.

Would like to see pictures of whole board, still likely to be worthwhile changing whole thing unless huge!
Hmmm its a 20 way (10 in each layer) unit. I recently had to change the Time clock in the lower half and that fitted OK without needing to do surgery on the opening. Just changing the items I listed in my initial post would cost around £60 plus VAT. I doubt I'd get a 21 way split unit with all the bits for that!!! Plus I'd probably need to replace some tails as the originals might not all reach with a horizontally split CU.

The full contents of the CU is:

Top row: 6 MCBs (Garage 45A, PV 30A, 4 lighting 5A), two spacers, double width RCD for external power
Bottom row: 4 MCBs (Cooker 40A, External Power 20A, Alarm/Time Clock 5A, External Lights 10A) , three spacers, triple width Time Clock

The MCBs in the top row are fed from the isolation switch
The MCBs in the bottom row are fed from a 100mA RCD

There's quite lot of height clearance above the rail - how much height do I need? It looks like RCBOs would go in w/o problem.
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In some cases, I have found old-model RCBOs on ebay. Sometimes you find new old stock.

But I have not looked for the old MK range you show

It would be an advantage to have access to an RCD tester if fitting used parts.
I suppose it could be done with new RCBOs, a new main switch, new busbar, and bits of tri-rated cable with the flattened ends, which could fit between the busbar and an RCBO.

But you would need some din bar as well to fit the RCBOs and main switch, and that din bar would need to fitted at the correct height (and depth) to line up with the consumer unit cover.

But the type tested aspect would be out of the window.

Personally, I'd fit a new consumer unit. Or a large enclosure next to the existing one with loads of stand-alone RCDs in it.
Stand back and take a wider pic to show the whole lot please.

It is so old that I think the renovation work will not be worthwhile.
I can only imagine this is some sort of skeleton board or something where replacement may be difficult.
At least it has a DIN rail.

I can't see that RCBOs could fit.

But an RCD could go in the lower half.

Nice to see clock connectors.
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