Problems with Axiom MCBs

[big chaz]

Hi, does anybody know what other brands of MCB (if any) will fit an axiom consumer unit? I need a C32 breaker but i cannot source axiom C curve breakers anywhere. The initial problem is my Plasma cutter trips the B32 breaker as soon as I touch the trigger, and i cant think what else the problem could be.

 

[pc1966]

@pc1966 I didn't see your answer. I think we are broadly in agreement. (I know L-N faults don't have to meet disconnection times but at least considering them seemed wise)

Very much so, but usually if VD met at MCB rating it will disconnect OK-ish. More of a worry if feed is fault-only and not overload then you need to check cable adiabatic for the worst Zs.

 

[pc1966]

Looking at the Hager tables, they don't offer any selectivity between 32A D & 63A B, and for 63A C it is only to 0.48kA.

So my guess about "both at once, and when you least want it" appears to be true!

 

[timhoward]

Looking at the Hager tables, they don't offer any selectivity between 32A D & 63A B, and for 63A C it is only to 0.48kA.

So my guess about "both at once, and when you least want it" appears to be true!

Yes, that is in line with my experience too. I've known 3 all trip together once in an extreme case involving a plumber and a mini-digger....

 

[big chaz]

As much as I don't like relying on RCD for disconnection times, if you find you can't meet the Zs for a 32A D-curve (something like 0.28 ohm from memory but CHECK THAT) then having an RCD would allow you to achieve < 0.4s for L-E faults (the only ones where ADS is specified).

As this ought to provide over-current protection for the cable you should have no other worries so long as it is meeting voltage drop.

Having separate RCD for those circuits would make sense as well, as I suspect a high chance of tripping the whole house (or half of it) otherwise!

The RCD only supplies the outbuildings so not much of an issue if it does trip, and there is plenty of natural light in the workshop otherwise I would be putting emergency lighting in. Surely 6mm NYYJ is plenty for 32a even with a d type breaker? It's only a few metres of cable to each socket

 

[pc1966]

The RCD only supplies the outbuildings so not much of an issue if it does trip, and there is plenty of natural light in the workshop otherwise I would be putting emergency lighting in.

Sounds fine then.

Surely 6mm NYYJ is plenty for 32a even with a d type breaker? It's only a few metres of cable to each socket

For overload protection then yes.

For fault protection it would also depend on the sub-board Zs which in turn depends on your supply Ze and the sub-main feeding that CU. Of course you can disconnect using the RCD to avoid the demanding final Zs value, but MCB are fare more reliable so I always feel a little uneasy going down that route.

Essentially your NYYJ R1+R2 would be added to your sub-main R1+R2 and your supply Ze and that total final circuit Zs has to be 0.28 ohm or less to meet 0.4s ADS (i.e. to hit the magnetic trip point at 20 * 32A).


Would you actually test your RCD regularly using the test button?

 

[big chaz]

Sounds fine then.


For overload protection then yes.

For fault protection it would also depend on the sub-board Zs which in turn depends on your supply Ze and the sub-main feeding that CU. Of course you can disconnect using the RCD to avoid the demanding final Zs value, but MCB are fare more reliable so I always feel a little uneasy going down that route.

Essentially your NYYJ R1+R2 would be added to your sub-main R1+R2 and your supply Ze and the total final circuit Zs has to be 0.28 ohm or less to meet 0.4s ADS.

Would you actually test your RCD regularly using the test button?

I know it's not ideal but I can't see a better way of making the plasma cutter work, when D type is the recommended fuse. I've checked the welder manual too now and it recommends a D45 or D63, no wonder it occasionally tripped the B32. I never use it on more than 3/4 power so the D32 should solve that issue too.

I do agree with what you are saying, but if my supply Ze is high there isn't much I can do about it is there? I will get it tested but I don't think there will be a resistance problem over 30ft of 16mm then 10ft of 6mm surely? So if the total Zs is high it will be the supply so I'll be stuck using the RCD anyway

 

[timhoward]

if my supply Ze is high there isn't much I can do about it is there? I will get it tested but I don't think there will be a resistance problem over 30ft of 16mm then 10ft of 6mm surely? So if the total Zs is high it will be the supply so I'll be stuck using the RCD anyway

There actually are things you can do. Fuses have higher max Zs values, e.g. a BS88 32 amp fuse max Zs would be 0.79 ohms. So if the total Zs is high best practise would be to at least consider if there are other ways to supply the equipment in order to allow automatic disconnection of supply to be via the over current protective device as opposed to an RCD.
But until we know the actual readings from your installation we can't really advise.

 

[pc1966]

I will get it tested but I don't think there will be a resistance problem over 30ft of 16mm then 10ft of 6mm surely? So if the total Zs is high it will be the supply so I'll be stuck using the RCD anyway

Depends. If it is 16mm 2-core SWA then R1 = 1.15 ohm/km, and R2 = 3.7 so for about 10m that is 0.049 ohm. If 3-core then R1=R2 = 1.15 so R1+R2 about 0.023 ohm.

For NYY-J 6mm it is about R1 = R2 = 3.08 so 0.019 ohms for 3m.

Working backwards from 0.28 for 32A D-curve we have:

• 2-core Ze max = 0.28 - 0.019 - 0.049 = 0.21 ohm
• 3-core Ze max = 0.28 - 0.019 - 0.023 = 0.24 ohm

Both are quite low. If you are TN-C-S you might meet them (assuming your PSCC at supply is a bit over 1kA) but probably not if TN-S (and no way TT). Measuring the current workshop CU Zs & PSCC would be the best starting point.

As @timhoward has already pointed out a fused-switch for 32A is much easier to meet, but check the manufacturer's data on what fuse(s) are acceptable.

If there is no way other than an RCD for meeting disconnection then one option for higher reliability, not the cheapest, is to have a 100mA delay RCD on the sub-main supply and a 30mA 'instant' for the socket outlet. That way you would still disconnect in under 0.4s even if one RCD was faulty. However, depending on how it comes from your main house CU that may (or may not) be easy.

You don't need the usual additional RCD protection on SWA cable even if it is buried or hidden in plaster (assuming armour is earthed as it should be and it meets 5s disconnection) so if you have a non-RCD 63A C-curve feeding a separate RCD enclosure to feed the sub-main, you could change that for a 100mA delay and then have a 30mA one in the workshop CU for shock protection.

 

[MJPD29]

Are you fitting this electrical equipment as a DIY project?

 

[big chaz]

Depends. If it is 16mm 2-core SWA then R1 = 1.15 ohm/km, and R2 = 3.7 so for about 10m that is 0.049 ohm. If 3-core then R1=R2 = 1.15 so R1+R2 about 0.023 ohm.

For NYY-J 6mm it is about R1 = R2 = 3.08 so 0.019 ohms for 3m.

Working backwards from 0.28 for 32A D-curve we have:

• 2-core Ze max = 0.28 - 0.019 - 0.049 = 0.21 ohm
• 3-core Ze max = 0.28 - 0.019 - 0.023 = 0.24 ohm

Both are quite low. If you are TN-C-S you might meet them (assuming your PSCC at supply is a bit over 1kA) but probably not if TN-S (and no way TT). Measuring the current workshop CU Zs & PSCC would be the best starting point.

As @timhoward has already pointed out a fused-switch for 32A is much easier to meet, but check the manufacturer's data on what fuse(s) are acceptable.

If there is no way other than an RCD for meeting disconnection then one option for higher reliability, not the cheapest, is to have a 100mA delay RCD on the sub-main supply and a 30mA 'instant' for the socket outlet. That way you would still disconnect in under 0.4s even if one RCD was faulty. However, depending on how it comes from your main house CU that may (or may not) be easy.

You don't need the usual additional RCD protection on SWA cable even if it is buried or hidden in plaster (assuming armour is earthed as it should be and it meets 5s disconnection) so if you have a non-RCD 63A C-curve feeding a separate RCD enclosure to feed the sub-main, you could change that for a 100mA delay and then have a 30mA one in the workshop CU for shock protection.

It's 3 core and the armour is earthed both ends so should be pretty low. I think a fused switch is a good idea, I will definitely look into it. Supply is TNCS so hopefully won't be required but we will see.

 

[pc1966]

It's 3 core and the armour is earthed both ends so should be pretty low. I think a fused switch is a good idea, I will definitely look into it. Supply is TNCS so hopefully won't be required but we will see.

OK, that is looking promising.

As a 32A socket outlet you need the 30mA RCD somewhere, just how it is engineered depends on the overall design. If it is at the sub-main feed already (as you said in post #10) then at the workshop you can use any very high-value breaker as a convenient means to feed the fused-switch while keeping distribution and overall isolation from whatever CU you end up with there. Certainly Hager or Schneider are good quality and they actually give to information to allow selectivity to be computed.

But...this is not how you would design an installation for this purpose!