Changeover for backup generator

[stillp]

Both.

Regardless, what about overcurrent protection?

 

[JohnW2]

Both.

I don't really get that. If it was a response to my first question, that would mean that you were agreeing with me that "ADS" does not encompass overcurrent protection. If it's a response to my second question, you're merely telling me that "ADS" was never called "EEBADS" in your machinery world.

Regardless, what about overcurrent protection?

I don't understand your point. An OPD will respond to an overcurrent purely on the basis of the current - e.g. a Type B MCB will operate within an hour at 1.45In - and that's not affected by the 'source impedance'. If the source impedance is so high that the current in the circuit never gets high enough to cause an OPD to operate, then there is no need for an OPD to operate.

Kind Regards, John

 

[stillp]

If the source impedance is so high that the current in the circuit never gets high enough to cause an OPD to operate, then there is no need for an OPD to operate.

Possibly, possibly not. We've debated this previously.

 

[JohnW2]

Possibly, possibly not. We've debated this previously.

I don't recall that. As I see it, if the OPD is appropriately rated to satisfactorily protect the cable, then that's it. If aspects of the circuit (or source) are such that it's impossible for an overload current to get high enough to damage the cable (hence also not high enough to make an OPD operate), then I don't see what the issue would be.

Kind Regards, John

 

[JohnW2]

... inc. VAT: £255.00

That seems an awful lot of money for a 100A C/O switch, a 40A RCD and a couple of indicator lights in a polystyrene box (and that plastic box may not be compliant with current Wiring Regs)

Kind Regards, John

 

[SimonH2]

As I see it, if the OPD is appropriately rated to satisfactorily protect the cable, then that's it.

That's what I'd have thought.
There seems to be this concern that in the event of a fault, the magnetic trip may not operate. If the supply impedance is too high for that, then the effect is similar to an overload rather than a fault - and the OPD will operate (or not) according to the current present. The cable and OPD doesn't care if the thing or things limiting the current to something over the nominal rating of the OPD but less than the magnetic trip level is upstream or downstream of the OPD.
Then we come to the issue of what happens to the genny if we overload it - and this seems to be a cause of concern in the minds of some when choosing the OPD protecting the downstream circuit. Well IMO that is down to the genny - if overloading it causes damage then that's a problem with the design of the genny and it's controls.

 

[JohnW2]

That's what I'd have thought. ... There seems to be this concern that in the event of a fault, the magnetic trip may not operate. If the supply impedance is too high for that, then the effect is similar to an overload rather than a fault - and the OPD will operate (or not) according to the current present.

I'm not really sure what you're saying here. Are you talking about the magnetic tripping failing to operate at a current which should cause it to operate, or a fault of sufficiently high (i.e. not 'negligible') impedance that the fault current is below the magnetic trip threshold? In either case, I'm not really sure what you are suggesting one can do about it.

Then we come to the issue of what happens to the genny if we overload it ...

In terms of the sort of genny likely to be used domestically, in my experience the usual response to an overload is for it to stop!

Kind Regards, John

 

[SimonH2]

Well in various discussions, there seems to be this concern from some that with a small genny, Zs is large enough that if there's a fault (ie a short circuit), the fault current son't be sufficient to operate the magnetic trip. Thus there's a dangerous situation where there's a fault but the OPD doesn't trip - either at all, or in a short enough timescale. Eg, some seem worried about the idea of having a smallish genny that's not capable of operating the magnetic trip in (say) a B6 MCB on a lighting circuit, and not capable of tripping a B32 at all.
This, IMO, is no different to having an overload with an impedance of a similar magnitude. The current in the circuit is higher than designed for, but not high enough to "instantly" operate the magnetic trip in the OPD.
Either the current is high enough that sooner or later the OPD will trip, or it's not high enough for that. In either case, the tables of cable size/installation method/OPD type/etc cater for that - the overload tripping characteristics of the OPD are factored into the permissible combinations such that no dangerous situation downstream of the OPD will occur. Neither the OPD nor the cables care whether the current is limited by a large Zs or a large Zload - current is current.

And a couple of times the discussion has turned to what the genny does when overloaded - with concern that lack of effective overcurrent protection can result in damage to the genny. IMO that's a matter for the genny and it's control/protection systems, not the overcurrent protection of the downstream circuits. Ie, if we are concerned about overloading the genny, then we should have suitable overload protection built into the genny - and if there's inadequate discrimination (my small genny isn't going to trip the B32 on the RFC) then we either accept that the whole installation is going to go off (I'll accept that as a tradeoff for keeping a few things like light, heating, internet) working) or we get a bigger genny where that's not a problem, or we design the installation in a suboptimal manner (for normal use) in order to accommodate the few occasions we want the genny.

If you are planning to use a genny for long periods then it might make sense to design the installation around it, but for occasional emergency use IMO it's enough to apply a bit of common sense and be careful what you try to use while on genny.

 

[bernardgreen]

If the Z of the genny is too high for a 32A MCB to trip on a fault then fit an MCB on the genny output that will trip on the fault current that the Z of the genny will allow to flow through the fault. The genny may already have one already fitted.

 

[JohnW2]

If the Z of the genny is too high for a 32A MCB to trip on a fault then fit an MCB on the genny output that will trip on the fault current that the Z of the genny will allow to flow through the fault. The genny may already have one already fitted.

I suspect that a problem with that could be that small gennys are simply not capable of supplying even remotely high currents (whether fault currents or otherwise). If that is the case, then installing an MCB which would trip magnetically at the maximum current the genny was capable of supplying would seriously restrict the continuous current that could be supplied during normal operation.

The answer surely is to earth-reference the genny output and use an RCD to clear faults?

Kind Regards, John

 

[bernardgreen]

restrict the continuous current that could be supplied during normal operation.

the MCB is fitted between generator and the change over switch and would not affect normal operation

 

[JohnW2]

... Either the current is high enough that sooner or later the OPD will trip, or it's not high enough for that. In either case, the tables of cable size/installation method/OPD type/etc cater for that - the overload tripping characteristics of the OPD are factored into the permissible combinations such that no dangerous situation downstream of the OPD will occur. Neither the OPD nor the cables care whether the current is limited by a large Zs or a large Zload - current is current.

Exactly. That's what I said a while back, as far as overload protect (of cables) is concerned. As you say, the tables we use effectively indicate what cable is 'safe' with what OPD. If aspects of the supply or installation are such that it is impossible for the current to become high enough to cause the OPD to operate, then that's fine - it simply means that the cable is always 'safe' and doesn't actually need the protection of an OPD.

Fault protection is a different matter, and I imagine that it will often be difficult/impossible to achieve it (with standard disconnection times) using OPDs when the supply is from a small genny. However, as I've just written to bernard, the answer is surely RCD protection?

Kind Regards, John

 

[JohnW2]

the MCB is fitted between generator and the change over switch and would not affect normal operation

I don't understand. The current being supplied 'during normal (i.e. non-fault) operation' (from genny supply) would have to pass through that MCB.

Kind Regards, John

 

[bernardgreen]

OK I took normal to be from the mains. The MCB between genny and changeover is selected to be one that wil trip magnettically on what ever current will flow from the genny, into a dead short and then back to the genny.

 

[JohnW2]

OK I took normal to be from the mains. The MCB between genny and changeover is selected to be one that wil trip magnettically on what ever current will flow from the genny, into a dead short and then back to the genny.

Indeed. However, as I said, if the maximum current the genny can supply is very limited, then an MCB chosen to magnetically trip at that current might restrict how much current could be supplied continuously during 'normal' (genny-supplied) operation.

Taking an example, if the maximum current the genny could supply was, say, 50A, then one would have to use a 10A Type B MCB to get guaranteed magnetic tripping at that current, which would mean that the maximum continuous current the genny could supply without (thermally) tripping that MCB could be as low as 11.3A.

I have no idea what sort of 'fault current' a small genny could supply during the first few tens of milliseconds, but, in my experience, any load appreciably above the 'rated' current will result in rapid reduction in output voltage followed by the engine stopping within a few seconds at most.

Kind Regards, John