Garage RCBO tripping large machinery
- Thread starter jmac80
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Thanks. That certainly seems to confirm that I1 and I2 remain the same across the whole range of MCBs. They do use slightly odd language, though - saying "Thermal trip point - 1.13 to 1.45 protector rating." (for Types A, B, C & D). Apart from not being sure what they mean by "protector rating", thermal tripping obviously continues beyond 1.45In - all the way up to where magnetic tripping takes over!
In passing, I must say that I don't think I've ever met, or even heard about, a Type A MCB.
Kind Regards, John
No, neither have I.
The text says for very sensitive circuits such as semi-conductors - never met one of those either.
The text says for very sensitive circuits such as semi-conductors - never met one of those either.
Ah, maybe that would be half of a conductor
However, one wouldn't expect electricians to have much to do with that, since they only use OPDs to protect cables, not equipment!!Kind Regards, John
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I thought a semi-conductor was the driver of a one man bus!
But the whole idea of the thermal part of the trip is the circuit is disconnected before any cables overheat. So if you use a D16 with 2.5mm² cable the trip will open before cable damage but with a B32 although it will trip with a lower in-rush current enough heat can be generated to damage cable before it trips.
So the whole question is if the magnet part of the trip is required if other factors like loop impedance stop excessive current?
There was a point when the single module width RCBO came out where they all seemed to be C rated.
But the whole idea of the thermal part of the trip is the circuit is disconnected before any cables overheat. So if you use a D16 with 2.5mm² cable the trip will open before cable damage but with a B32 although it will trip with a lower in-rush current enough heat can be generated to damage cable before it trips.
So the whole question is if the magnet part of the trip is required if other factors like loop impedance stop excessive current?
There was a point when the single module width RCBO came out where they all seemed to be C rated.
Indeed - which is why protecting a 2.5mm² cable with a 16A MCB (any Type) is compliant with regs, but protecting it with a 32A MCB (any Type) isn't compliant.
The primary reason for needing the magnetic part is to achieve required disconnection times to provide protection against electric shock. If "other factors like loop impedance stop excessive current" it becomes more difficult (or impossible) to achieve those required disconnection times.
There was, and Type Cs are still very commonly seen, and I never really understood the reason (particularly for domestic use). Many/most wholesalers list Type C RCBOs up to C50, yet (as discussed earlier in this thread) I would have thought that few domestic installations would have circuits with a low enough Zs to support Type C's anything like as high-rated as 50A.
Kind Regards, John
Not sure about that, Unlikely that any electric shock fault current ( that through the body ) would trip a magnetic 6 amp MCB.JohnW2 said:
More likely is that the magnetic trip will operate when a fault causes a high current arc that creates a localised very high temperature hot spot with the risk of a fire being ignited. Without the magnetic trip and thus relying on the thermal trip in the MCB then the arc could easily be burning long enough to create a fire before the thermal trip operates.
Devil's Advocate: I would suggest "isn't always" would be more appropriate than "isn't".
Of course not - but that's not the point. We are talking about the requirements of Chapter 41 ("Protection against electric shock") of the regs. ADS (i.e. achievement of specified disconnection times in the event of a fault between L and an exposed-c-p) is the primary means of protection specified in that chapter. The point is obviously that the fault is required to be cleared very rapidly (within 0.4s with TN, 0.2s with TT), hopefully before anyone has a chance to touch the live exposed-c-p (and simultaneously touching something 'earthed').
Kind Regards, John
I know that Devil's Advocates 'scrape barrels', but I'm not sure what you have in mind here. Even with installation Method E ('free air etc.), 2.5mm² only has a CCC of 30A, so I'm not too sure what situation(s) you have in mind in which having an OPD with an In of 32A as the sole protection of that cable would be compliant. Of course, if you were going to argue that the load 'could not create an overload', then there would theoretically be no need for overload protection at all (although fault protection would still be required), but that'a a bit different. Whatever, this is a DIY forum, and I would suggest that you probably should not potentially confuse DIYers by 'trying to be clever' in this way.
Kind Regards, John
And I would suggest you stop patronising, verging on lying, to DIYers because it's easier for you to do that than to be honest and accurate. What are you - a plumber?
That would be a better form of words.
No.
However, I would try not to put into the minds of DIYers the idea there are situations in which it would be acceptable to protect a 2.5mm² cable with a 32A OPD, even if there are some situations in which those with expert knowledge and the ability to confirm that fault protection was adequate, might decide that no overload protection was required. If you want to call that 'verging on lying', then fair enough - I would prefer to be guilty of that than of putting potentially dangerous ideas into to minds of those who did not fully understand.
Kind Regards, John
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