ZS for an motor run off an inverter

It is not I who has completely flipped Chris, it is simply that the scope definition is much too wide. I thought you had learned the regs too, but then perhaps Chapter 1 is not very interesting!


I am simply trying to summarise the very difficult situation faced by inspectors, it is not up to them or the client to define the limitations under normal conditions, this should be defined by the book of regulations to be used for the inspection. It is not reasonable to expect one to know all the details of the machinery regulations, and standards for cookers and other appliances or anything except BS7671. To make this work the boundary needs to be properly defined, the end of BS7671 (fixed wiring for now), and appliances or attached items that cannot be inspected to BS7671. This includes the totality of connected machines, control panels, fire alarms, etc. and the situation can only get more complex when more technology enters the world. Take a domestic electronic control system using wi-fi for control. It controls lights, AV systems, heating, air-con, etc. It contains loads of electronics. All the loads are wired in. Now, what do I inspect and test? One could probably break the entire expensive thing with an MFT and some misplaced testing. Most electricians would wisely run away. The definition should stop at an Isolator, a socket, or the first electronic item encountered (dimmer switch thermostat etc.). Zs or whatever cannot be safely tested, but in the grand scheme of things, does it matter? We can check the Earth bonding and this is quite sufficient, there will probably be RCD protection anyway, and overload is not at all likely. Similarly, disconnection time is probably the least of our worries.

The definition should stop at an Isolator, a socket, or the first electronic item encountered (dimmer switch thermostat etc.)

So who (or if you prefer which standard) takes repsonsibility for the 230V cable between the dimmer lightswitch to the light and the connection to the light fitting itself? I'd hate not to be able condemn twisted & taped joints or use of bell wire or lack of earthing to a class I light fitting because it was out of scope of BS 7671.

    - Andy.

but if the wiring to the light came in 12V and the 'switch' was also involving some  box of electronics providing bell-transformer like insulation then bell wire might be correct. (as indeed some elv LED drivers almost do) I should hasten to add I  fully agree the mains wiring to  mains lamp from a mains dimmer should be mains rated cable, but it is quite hard to draft a regulation that correctly covers both cases. And does not encourage folk to ask for a Zs test at the lampholder.

I think this thread highlights the conflict between the engineer, the consultant, and the lawyer, a single person may be all three, in which case there must definitely be a conflict of interest in the same way as that as a pharmacologist who becomes a GP.

All the engineer wishes for is a clearly defined and yes prescriptive set of rules to follow where nothing is left in doubt. If such a guide existed, there would be no room for doubt, but since that doesn't make money, things are left deliberately vague and indistinct. The best example of which is the 3 inspections from 3 different inspectors on the same installation and with 3 different results and conclusions, one man's catastrophic non-compliance is another's  'pass' but with a bit of remedial work required. The third most likely couldn't make up his mind, and just failed everything on the strength of a quick visual to be on the safe side.

I think that whjohnson has been a little harsh. Why do we have the judiciary? Surely laws are clear! The fact is that they are not as clear as the folk who draft them for Parliament would wish: such is language and it applies to BS 7671 as well.


The other issue is that different assessors find different facts on the same evidence. Such is human life!

The law has the same problem, many of them are not clear, and are then subject to interpretation by the courts. As we do not wish to involve lawyers or courts any more than is necessary, a clear guide is needed.


I agree that the light dimmer example is a bit poor, but there is a dilemma as to whether to IR test properly or just test between LN linked and Earth. In fact, light dimmers will generally allow Zs testing without problems, but some devices will not, particularly VFDs, etc. which completely reform the mains supply. The same may be said for UPSs, and battery to mains inverters. These will all become more common with "green" supplies, but clearly BS7671 is not equipped to cover them. Power electronic devices have to deal with overload situations because semiconductors are often known as the fastest fuses known to man! Because the active areas are very small, the rate of heating is very high, and heatsinks do not help because that heating is adiabatic, and we all understand that. This means that transient high current must be avoided very quickly indeed, and this is done by switching the power devices off as fast as electronically possible from real-time current measurement. If a fault causes a power device to fail short-circuit then the normal CPD will provide protection to the cables, which should be sufficient for safety.


We need to look at why disconnection time is important and realise that we are not analysing shock protection, it is fire protection which can be much slower. It is true that some shock risk can be found under some circumstances, but these can be removed by equipotential bonding if considered significant. If 5 seconds is considered safe for submains, why is it unsafe for installed motor drives? Really it only matters for Portable appliances which may develop live-to-case faults in circumstances where other Earth potentials may be present, for example, a kettle in a kitchen. Here the use of RCDs as additional protection is a good idea, but is this required in a factory full of machine tools where they are unlikely to work properly? In this case bonding to nearby machines is easy, and very effective. Blanket RCDs everywhere is not the correct Engineering answer to this problem of electronics everywhere. Many modern tools have 6-10 VSDs in the control system, 30mA RCDs are not the answer.

I would say that there is a good chance that this is outside the scope of BS7671 anyway.


110.2,xi