If the RCBOs are in a UK style consumer unit (i.e. meeting annexe ZB of BS 61439-3 in standards speak) supplied by a protective device specified by the manufacturer (typically a BS 88-3 100A or less (or BS 1361 equivalent)) the assembly will be rated at 16kA and so you should be fine. (See reg 536.4.201).

   - Andy.

Use a consumer unit approved for use in the UK, and don't mix and match components inside from different manufacturers.  You're relying on the manufacturer of the CU to declare it's safe for a very brief 16kA overload.

What is the Zs at the DB? Even with fairly short tails, it must be lower than Ze. Then after the RCBOs, is there really a prospect of a dead short close to the CU?

How are you measuring the Ze? What is the accuracy of your meter? I take the point, but when Ze gets so low (and PFC so high) you might need a specific low resistance ohmmeter rather than an MFT.

It's a hager fuseboard with hager 6ka rcbos in guys

Not really, it is better to consider that you are relying on the 100A company fuse to open first and thus limit the down stream energy reaching  the breakers and protect them for the very small fraction of real faults (if any, ever) that draw the full PSSC. Almost no real fault is zero ohms (if it was there would be no heat light or sound as no energy would be dissipated), and also within a meter or so of the CU so not limited by the resistance of some outbound cable....

The risk arises not so much from mixing MCBs, but rather more if you were to bypass the company fuse with bits of coat hanger wire - which I hope you don't.

Mike.

goldenboy1818 said:

It's a hager fuseboard with hager 6ka rcbos in guys

Domestic Consumer Units made for the UK market should meet annex ZB of BS 61439-3, which has extended tests with 16 kA fault currents to permit 6 kA or 10 kA protective devices to be used with fault currents up to 16 kA. The reason for this, is that it's taken that fault currents in domestic premises can be up to 16 kA for 100 A single-phase supplies.

If you have any doubt about this, you should check with the technical helpline of the manufacturer in question.

As others have said, though, it's important to only use protective devices endorsed by the manufacturer in UK consumer units, because of these enhanced 16 kA fault current tests.

this makes me wonder why they mark them as 6KA at all? no manufacturer would approve of their MCB in another’s enclosure , or does the consumer unit itself play a part in containment of the fault current?

this makes me wonder why they mark them as 6KA at all?

The can go in things other than consumer units - where their individual breaking capacity is relevant -  e.g in distribution boards, or caravan hook-ups or equipment (e.g. larger UPSs) or just a random modular enclosures; or may be fed by something of a higher rating than a BS 88-3 fuse. Yes, one option the CU manufacturer has is to use the enclosure to contain arc products if the MCB can't, to achieve the 16kA rating (but there are other options too).

   - Andy.

Newfutile said:

this makes me wonder why they mark them as 6KA at all?

The range may well be used in other applications than the domestic CU, but fundamentally unfortunately the breaker standard requires the marking.

I would interpret the marking as 'Usually suitable for 6 kA, but if installed (in accordance with manufacturer's instructions) in a DBO (consumer unit) that it has been tested in to the UK national annex, and protected by an upstream BS 88-3 or BS 1361 fuse up to 100 A, it is suitable for 16 kA.'

If we take the PFC to be 8 kA at the DB, and let us suppose that the sparks failed to protect the cable e.g. with a grommet and it has chafed so that a 8 kA fault occurs and the RCBO survives, what then? Must the RCBO be replaced? I would suggest that it should because its rating has been exceeded. (A 10 kA RCBO could remain in service.) So there is a reason for marking the RCBO even when supplied as part of a populated DB.

Frankly, the whole scenario seems most unlikely.