You might equally well ask whether it is safe to supply a SP circuit from a TP DB; or what about for example, a lathe with a 3 phase motor which is not running, but a SP lamp and DRO which are.

I don't see a problem.  Graham's caveat concerning RCD protection is entirely reasonable, but even then, taking one phase off a 3 phase plug isn't any different from using a BS 1363 plug in a circuit which (due to age) has no RCD protection.

Perhaps the OPs fear is the commonly held mis-belief that taking a single phase load through a three phase RCD will lead to it tripping....

Chris Pearson said:

taking one phase off a 3 phase plug isn't any different from using a BS 1363 plug in a circuit which (due to age) has no RCD protection.

Well, yes ... and no.

The issue is two-fold simply because it's a place of work:

(a) risk assessments need to be reviewed when standard change ... and this has been changed for a very long time, so the duty-holder should be involved in the discussion.

(b) More importantly, anyone installing, manufacturing or supplying equipment for the workplace will have to comply with Section 6 of the H&S@W etc. Act, and quite possibly the CDM Regulations, all of which put the onus on the installer (or designer if part of the electrical installation) to ensure safety ... i.e. that likely makes consideration of the lack of RCD your (the installer, or person fitting the plug) problem. To quote Section 6(3) of the H&S@W etc. Act 1974 (as amended):

(3) It shall be the duty of any person who erects or installs any article for use at work in any premises where that article is to be used by persons at work or who erects or installs any article of fairground equipment to ensure, so far as is reasonably practicable, that nothing about the way in which the article is erected or installed makes it unsafe or a risk to health at any such time as is mentioned in paragraph (a) of subsection (1) or, as the case may be, in paragraph (a) of subsection (1) or (1A) above.

Agree those are the rule,  but it is hard to see how removing the ability to unplug something and making no other changes makes it any safer to maintain. There are many bits of kit where a 30mA RCD would be up and down like the proverbial. This is where the documented exception and RA come into play - and for more or less fixed kit. There are things one can do (double earthing is one) to remove the single fault to danger that  is the ' what if the CPC comes detached in the plug' that is the big worry that makes high leakage kit without an RCD dangerous. It rather depends how the plug and socket are used - if the lead is trailed across a  car park and pulled in and out every day, it is far more of a worry than a tidily mounted plug socket pair on the wall at waist height obviously isolating  an adjacent  milling machine or something that is unplugged only when the man is called out and the machine is being repaired.

Mike.

I can see that a risk assessment for inaccessible sockets or those in restricted areas might permit the absence of RCD protection, but not where an ordinary person might plug in a piece of office equipment, a portable power tool, or a vacuum cleaner, etc.

So how long do we allow for a building to be upgraded?

Now I am going to struggle not to take my tester in to work (in a government building). It might be interesting to see the response to a tripped RCD.

I can see that a risk assessment for inaccessible sockets or those in restricted areas might permit the absence of RCD protection, but not where an ordinary person might plug in a piece of office equipment, a portable power tool, or a vacuum cleaner, etc.

So how long do we allow for a building to be upgraded?

Now I am going to struggle not to take my tester in to work (in a government building). It might be interesting to see the response to a tripped RCD.

Chris Pearson said:

I can see that a risk assessment for inaccessible sockets or those in restricted areas might permit the absence of RCD protection, but not where an ordinary person might plug in a piece of office equipment, a portable power tool, or a vacuum cleaner, etc.

Well, even then, you need to demonstrate that the risks are limited (e.g. access to the equipment powered is also limited, and/or some other precautions taken) ... see my previous post.

Chris Pearson said:

So how long do we allow for a building to be upgraded?

Not the point, the issue is that a new machine is being connected up, so the question gets asked at that point, 'Do we fit an RCD now as required by the current version of BS 7671, or are there other mitigating measures in place that means it's safe so far as reasonably practicable'

Of course, if the manufacturer of the machinery recommends or requires RCD protection, you've not really got a leg to stand on in any case, under Section 6 of H&S@Wetc.Act, and PUWER.

gkenyon said:

Chris Pearson said:

So how long do we allow for a building to be upgraded?

Not the point, the issue is that a new machine is being connected up, so the question gets asked at that point, 'Do we fit an RCD now as required by the current version of BS 7671, or are there other mitigating measures in place that means it's safe so far as reasonably practicable'

With respect, I beg to differ. Surely the time to do the risk assessment is when the socket is installed? The assumption at that time ought to be that anything could be plugged in unless access is restricted.

The alternative is that whoever plugs in equipment must do a risk assessment, which wouldn't suit Mrs Mop with her floor buffer.

Perhaps the middle ground is when periodic I&T is undertaken? There is no reason why a factory owner should be aware of a change in BS 7671 until then. Of course, if the factory owner ignores the EICR, and injury arises, then he (or she) can fully expect a hard time from the victim's lawyers.

And this rather depends on the type of socket. The cleaner will not want a 32 A 5 pin socket for a floor polisher.  

A 13A socket and perhaps a 3 pin 16A CEEform single phase, "caravan plug",  the the assumption that almost anything may be plugged in is pretty good,

Less so for the socket that is on dedicated radial and is very clearly only intended for the adjacent large lump of 3 phase machinery that screws to the floor.

Enter the deviation ...

Mike.

Chris Pearson said:

With respect, I beg to differ. Surely the time to do the risk assessment is when the socket is installed?

Yes ... but that risk assessment might not be able to foresee use (or protection requirements) by a later machine ... AND the risk assessment should be reviewed when standards change.

In a workplace, we can't assume all socket-outlets are "general purpose".

Think about it ... this thread asks specifically about a piece of work equipment., in particular a machine.

Then have a look at Section 6 of the H&S@W etc Act, 1974, and the Provision and Use of Work Equipment Regulations, and the Supply of Machinery (Safety) Regulations ... I'm sure nowhere in that legislation says you can omit some of your responsibilities just because there's a socket-outlet that's convenient to use, without checking that socket-outlet has the relevant provisions in place (like the correct overcurrent protective device ratings, and RCD, as applicable)?

Yes, you could (unless the manufacturer requires one) risk assess that an RCD was not necessary (if you felt you could stick your neck out that far depending on what else is in place) ... but to simply say "go on, just connect it" without checking these things is surely irresponsible in a discussion forum on a professional engineering institution's web-site, given what the legislation actually says?.