I do understand that the RLV is required for constructions sites

I'm not sure it is, at least from a BS 7671 perspective. It's an option certainly and in most circumstances a very good one - but there are others - including LV plus 30mA RCD protection. Most of the 230V world manages with 230V hand held tools and RCDs (a bit like fused plugs, RLV is rather UK peculiarity). The HSE might well prefer RLV too, but we're getting a bit outside BS 7671 then. Worth a mention to the customer certainly, but be aware that there are other possibilities.

   - Andy.

Thanks Andy.

Yes, at the moment it does fit the alternative of LV plus 30mA, but the fact that someone has gone to the trouble of fitting a 5kVA transformer and 110v socket outlets makes me think that the intention would have been to go for the 55v to Earth option (CTE) and the protection against shock that that brings?

Quite possibly - or they might just have wanted the ability to use the same tools they use elsewhere on RLV systems. Certainly worth raising with their safety elf.

One other thought - is it definitely a single phase transformer? There is a 3-phase version of RLV that has 110V between lines and therefore around 63V L-PE - which might account for you seeing something well in excess of 55V L-PE. Off load it might be somewhat higher than 63V - although I wouldn't have expected it to be that close to 110V.

   - Andy.

Thanks for the follow up. It is definitely 240v single phase and the Line to Earth is close to 120v off load. I get your point that, maybe they 110v that is used on construction sites where there is RLV, and would want to use it in their away from site premises, but for the minimal difference in cost why would you not specify a CTE transformer with the enhanced safety benefit, I can not see any practical reason?

We have wired transformers in that way to test kit that was due to be delivered to the states, where mains is 110-0-110 more or less and it avoids a need for double pole ADS - fusing the 110V live will do.

I agree it seems a bit odd in the case you describe.

Mike.

BS 7671 permits RLV to be used anywhere, but it's only mandated on construction sites.

According to BS EN IEC 60209-2, there is really nothing to prevent the 4h-keyed yellow plugs and socket-outlets from being used on either system.

This would really be an observation to discuss with the duty-holder, as it's their H&S / electrical safety risk assessment at the end of the day. I agree that the system does not offer any additional safety over a 230 V supply at present, and so it's a fair question to ask. However, unless 10 or 30 mA RCD protection has been omitted (which you don't discuss yourself, but has been mentioned by others) it does not appear, at least from the information provided in this thread, to be a non-conformity with BS 7671.

However, if 30 mA or 10 mA RCD protection is not provided for socket-outlets with rating not exceeding 32 A, I would definitely consider providing a coding.

I agree that this sounds very odd and consider that it MIGHT be a mistake.

Having gone to the trouble and expense of fitting a transformer with a center tapped 110 volt secondary, it seems very odd not to wire it in the conventional way with the centre tap earthed.

The only extra expense would be for double pole MCBs to protect the 110 volt circuits.

broadgage said:

I agree that this sounds very odd and consider that it MIGHT be a mistake.

Definitely ... I know a lot of large companies have "electrical procedures" that mandate that all non-battery electrically-powered single-phase power tools are supplied by a safety transformer with a 55-0-55 V centre-tap-earthed secondary.

In which case, this does not conform to the company's safety procedures (and could mean the company is exposed to the possibility of a HSE prosecution if there is an incident).

It's therefore absolutely imperative that it's discussed with the duty-holder!

AJJewsbury said:

I'm not sure it is, at least from a BS 7671 perspective.

It is, for LV circuits on construction sites, where protection by RCD with rated residual operating current not exceeding 30 mA is omitted, supplying both socket-outlets, and hand-held equipment, having current rating up to 32 A ... see 704.410.3.10. Other options include electrical separation for individual items of equipment, or SELV/PELV.

But since 30 mA (or less) RCD protection for the socket-outlets is ordinarily required by 411.3.3 (unless RLV is used), it seems that the RCD protection would be the deciding factor for the 110 V LV circuit?

broadgage said:

The only extra expense would be for double pole MCBs to protect the 110 volt circuits.

Unless OCPDs on the primary can meet the requirements?