ZS for an motor run off an inverter

David, shock protection is the "time to achieve a voltage considered safe" in 419.2


The problem we have, is that manufacturers are inventing some really great stuff, but sometimes failing to consider how protection is achieved in accordance with IEC 61140 ... or at least communicate that so designers can consider whether any additional protection is necessary.

With RCD, you can test before the drive, and then earth continuity downstream - this is common practice and acknowledged in BS 7671.

What happens with a dimmer switch though? I think you are all making it too hard. You wouldn't necessarily carry out Zs downstream of a dimmer, for fear of damaging it.


If the inverter does not offer voltage collapse (crowbar) protection, it's just like a big dimmer switch.


So, you take Zs before the inverter (dimmer switch or electronic switch) and R1+R2 downstream.

This is part of the same problem with do you 'PAT' equipment that is screwed to the wall, and when is fixed wiring part of an appliance.

The honest answer is that  it is not always clear - the extremes are clear, we do not do an EICR inside a self-contained item like a  washing machine, nor do we treat the light switch as a portable appliance.  The grey areas arise with equipment that is spread out - in the home the central heating system is one, that is sort of fixed wiring, but the voltages and currents between thermostats tanks and boilers may be 'mains level logic' or DC, controls to pumps may be via a boiler that puts a triac in the neutral to switch it.

Problems also arise with things like stage lighting, where gear in racks may well plug in but is not really an appliance, and the mains flows though it to somewhere else via more fixed wiring.

I kind of agree that BS7671 should wind it's neck in from areas where it clearly makes no sense, but right now that is not how it is written (and I have a few comments for the DPC that point to that as well).

But there cannot be a hole where no-one knows who is responsible.

For now I suggest that the practical approach for the OP is a bit hybrid, and is to measure the Zs up to the point it stops being normal mains (presumably the input to the inverter). However verify R₂ (earth continuity) all the way to the motor body (and if the inverter cable is screened this should be solidly earthed). And take a look at the inverter instructions and see that there is nothing else that should be done.

Take care that some inverter designs have large capacitors on the DC bus, and one of the many things that can happen in a fault is that the capacitor remains charged long after the input side ADS has operated. (by large in this context I mean that 1/2Cv² is more than a couple of joules and the stored energy is therefore potentially lethal)

regards

M.

The advice I got on this subject by a drive specialist a number of years ago was to forget about the VSD and design the circuit as if the drive wasn’t there. Any earth fault downstream of the VSD would be handled by the VSD itself. An internal fault in the VSD upstream of its own protection would be handled by the CPD. We were installing the units in a fertiliser factory with the VSDs quite some distance from the motors they controlled. We just did a loop to the VSDs and an R1+R2 to the motors from the VSDs and ensured that the total would be low enough to take out the CPD. Hardly a sophisticated approach but acknowledged as satisfactory by our NICEIC engineer.

With regards to checking Zs on an existing VSD arrangement, I doubt that bypassing the VSD is a practical consideration. Even checking R1+R2 from VSD to motor might be totally unnecessary and might only be worth the effort if access to the motor terminals was being considered for other reasons. Otherwise confirmed to be earthed using a wandering lead seems an optimal approach.

Don't disagree Lyle, but BS 7671 requires you to address this now (well, since 2018), if the drive-to-motor connection is part of the electrical installation (Regulation 419.2)

It would be useful Graham if you could produce a document showing a manufacturer who has these instructions? Manufacturers do not have to comply with BS7671, which covers fixed wiring installations only. An inverter will remove output on a short to Earth very quickly indeed, as an act of self-preservation, probably within a fraction of a milli-second. In what way does this not comply with BS7671? Verifying this kind of thing is very difficult for a typical Electrician, so you are saying that he may not install this equipment? This is the problem with regulations and standards of all kinds, "features" get added which have no place because "someone" thinks it is either necessary or would be nice, and everyone else nods in a SAGE way and lets it pass. It doesn't matter if the idea comes from Cenelec or elsewhere, if it doesn't make sense to practicality, it shouldn't be there. There are many other examples to be found, there is no shock risk at all from an Earthed motor with properly installed wiring, with whatever mechanical protection suits the environment. This is a case where no additional shock protection should even be mentioned. 419.2 is not necessary at all. It now seems to be thought that the Earth conductor is considered "dangerous" in itself and that all the Earthing and bonding is inadequate unless additional protection is added. This is ridiculous and has no reasonable danger scenario which is improved. Someone may like to make the case of why it is required, preferably generally, and not a motor in the middle of a lake used for swimming with convenient handrails attached. It is fundamentally impossible to fully comply with 419.2 if the inverter is not an appliance, which in my opinion it is, therefore not covered by BS7671 at all. Should it be covered, the list of applicable standards to BS7671 is woefully inadequate, particularly as all the reliability of electronics ones are required (and basically physically unverifiable), and the definition of an appliance needs considerable revision to cover anything which does not have a plug. BTW this would simply mean that all machinery would be fitted with a plug (we would need some bigger kinds to be designed) and this would actually reduce safety as the Earth conductor could become disconnected! A plug for a 10MW motor inverter could be quite interesting!

BS7671, which covers fixed wiring installations only.

That hasn't been the case for a long time. A less inaccurate approximation would be that BS 7671 covers all parts of all LV and ELV electrical installations unless 'out-scoped' by some other more specific standard.

   - Andy.