Does SPD MCB size matter?

[oscar21]

Does this matter? I've fitted a Fusebox consumer unit and I didn't have enough 32A MCB's but too many 6A's. The SPD is fed from a 32A one, what's the deal with swapping them around. They are voltage operated not current operated so should be fine right?

 

[nicebutdim]

Yes, Like I say, if its private work we fit the Fusebox ones and if its council we fit also fit the Wylex ones with SPD's, Edwardes have a good deal on the wylex ones at the moment.

Off track slightly but the new Wylex stuff is pretty crap, you can't remove the din rail so have to take everything to bits just to get the RCD's out, they are exactly the same as the new Crabtree stuff as well so it all comes out of the same factory. And those metal blanks that slide about all ove the show when you are trying to get the lid on, woeful, although on the last board I did it came with different blanks that could clip in from the front.

Metal blanks can be a pain, but I understand they're used not only because it's a 'non combustible' material, but also because they require a tool to remove, whereas traditional blanks can be pulled out easily.

Wylex make blank modules that clip on the din rail and these also fit Crabtree starbreaker boards.

 

[loz2754]

Wylex make blank modules that clip on the din rail

As do Fusebox and Hager. I only use this type now.

 

[nicebutdim]

As do Fusebox and Hager. I only use this type now.

Some can be a pain to remove, but for very little extra money they look like a finished job.

 

[pc1966]

Does it really make that much difference if a 6A or 32A MCB is on the receiving end of a bolt of lightening?

Yes, but not for the obvious reasons.

As already mentioned most SPD are not rated to deal with a direct strike. The 'type 2' that most folks fit are intended for the 8us/20us waveform typical of induced lighting surges and similar when a strike hit something else and the power cables pick up the radiated EM disturbance.

They 'type 1' (or more commonly type 1+2) are rated for a portion of a direct strike, so if your building has a Lightning Protection System fitted then the assumption is a portion of the direct hit goes via the LPS to Earth, but some of it goes via the power cables and then the SPD has to divert it. This may have similar peak current ratings in the 10-50kA region but now the waveform is a 10us/350us shape (i.e. falr longer tail of decaying current) and as such it has about 17 times higher energy for a given current peak. That is why those SPD are often double-width and 5 times the price!

Back to the MCB, the problem here is inductance as at high frequencies (and the bulk of lightning power is around 100kHz, not 50Hz) that dominates circuit impedance over conduction resistance in most situation. That is why the regs have the 0.5m / 1m max cable length when fitting an SPD otherwise the series impedance of the cable loop increases how much voltage passes by to the supposedly protected installation.

When comparing a 6A MCB and a 32A MCB the "instant" magnetic trip point (instant in human terms, which is still several milliseconds, not microseconds) the trip point is just over 5 times higher, so your trip coil needs (for assumed same mechanical arrangement) 5 times the number of turns so the fault current applies the same force to initiate tripping. But inductance is proportional to turns squared, so your impedance to high frequencies is over 25 times higher!

 

[Flanders]

A 6A would possibly trip well before the SPD had a chance to operate, rendering it useless. For the cost of another MCB…

As for RCBO vs dual rcd… it’s wrong to say he should fit an all RCBO board. There’s nothing in the regs preventing using a dual rcd….. and it might be all the customer can afford.

See Reg 531.3.2 unwanted tripping (ii) rcbos should be considered. Also see section 314

 

[oscar21]

See Reg 531.3.2 unwanted tripping (ii) rcbos should be considered. Also see section 314

The way I read that is if you expect tripping then minimise it. Under a normal situation in a domestic setting you don't expect any RCD to trip whether its an RCBO or dual RCD board. However if you have a bank of computers that have a bit of earth leakeage then you might get unwanted tripping so use RCBO's instead.

 

[Flanders]

The way I read that is if you expect tripping then minimise it. Under a normal situation in a domestic setting you don't expect any RCD to trip whether its an RCBO or dual RCD board. However if you have a bank of computers that have a bit of earth leakeage then you might get unwanted tripping so use RCBO's instead.

The reg states it just something to be considered. So its up to you if you choose to follow it. I prefer to use separate rcbos, but if the spec requires a split load board thats what will be installed . Just to add regarding spds. Hagers connect straight to the main switch with no OCD in the board

 

[oscar21]

The reg states it just something to be considered. So its up to you if you choose to follow it. I prefer to use separate rcbos, but if the spec requires a split load board thats what will be installed . Just to add regarding spds. Hagers connect straight to the main switch with no OCD in the board

So do the Wylex and Crabtree ones, the first one I did I put the earth in the bottom and then thought why is there an earth position in the top as well. I'd set the board up with a space to the right of the main switch as well so it was all a kerfuffle to put it right again. I wish manufacturers would all stick to a similar design.