it sounds like it uses the concept of an IT system so that it keeps going in the event of first fault. An MCB or fuse will still protect against overload,  but of course cable damage or other fault grounding either side of the supply will not trip anything, nor will it pose any danger. (well, until a second failure grounds the other pole anyway ! )

Normally the two poles would be insulation tested with respect to the case of the kit at installation time- which may well be metal and earthed,  but with this sort of supply it is not really working as class 1

As IT, .It would be normal to monitor the offset voltage to ground, and alarm if there was a serious insulation fault. You do not say what is being supplied with this supply or if the load and supply are part of a pair or if they are from different suppliers - this sort of thing needs both ends considering together.

It is almost certainly safe unless there is something else wrong with it, - many small generators and inverters are also wired this way even with 230V 50Hz out but you are right to check and the suppliers/ installers should be able to explain.

115v 400Hz sounds aircraft like - is it the sort of kit that may be used in one ? shore power to aircraft can be a bit odd, as for obvious reasons, currents through the frame that may ignite fuel etc are a big no-no, so earthing once, and avoiding loops results in things that look a bit  odd.

Mike

ADS has been assumed ... but does the system and connected equipment use something else ... double or reinforced insulation, for example?

Agreed - or even separated - either for one item (see BS 7671 section 413)  or several (section 418.3).

Is whatever is connected to the secondary side of the PSUs fixed? If it's just a socket outlet that anything could be plugged into, the secondary side might not come under BS 7671 anyway (like a lot of smaller UPSs) but come under some equipment standard that may have its own ideas about shock protection (the basic principles are likely to be similar to BS 7671's but sometimes the significant details can vary). I've a feeling that 400Hz is slightly safer than 50/60Hz from a shock perspective too - which again might influence some details.

   - Andy.

AJJewsbury said:

or even separated - either for one item (see BS 7671 section 413)  or several (section 418.3).

Possibly ... I was also thinking that we don't know what these supplies do, and therefore whether they are part of the electrical installation (under BS 7671) or machinery as defined in the Supply of Machinery (Safety) Regulations (under BS EN 60204-1) ... but that doesn't really affect the classification.

RichHaze said:

We have gone back to the OEM to ask how their system design meets the intent of the wiring regulations for protection against electric shock but we are not getting an answer

If you take the view that this information is necessary to safely maintain the equipment, and check it continues to be safe, the perhaps Section 6 of the H&S@W etc Act 1974 requires it to be provided. Check out 6(1A)(c).

If your company's legal expert agrees, they could try sending an appropriately worded letter if you continue to get no response.

Hello,

Thank you for your replies.

gkenyon said:

ADS has been assumed ... but does the system and connected equipment use something else ... double or reinforced insulation, for example?

ADS is assumed because the 400 Hz power supply feeds multiple items of class I equipment and the exposed conductive parts of these items are not isolated from the exposed conductive parts of other circuits and also have fortuitous connections back to facility PE via the local metalwork which is connected to the MET.

AJJewsbury said:

Is whatever is connected to the secondary side of the PSUs fixed?

The PSU output circuit breaker feeds fixed 400Hz distribution boards (fixed installation) and these have individual fuses for the final circuits.  Final circuits then feed out to break out "patching panels" (fixed installation) on our facility and these connect to current using equipment on the units under test (UUT).  The UUT are not fixed and get swapped out for each test and our technician teams rig and patch the equipment using mil spec connectors and the like.

gkenyon said:

I was also thinking that we don't know what these supplies do, and therefore whether they are part of the electrical installation

As above - these feed aircraft OEM parts on the UUT.  However, in order to make the connections to the UUT our operators are in proximity to the earthed metalwork of the test rig, as well as the fortuitously earthed exposed conductive parts of the UUT (fed by the ungrounded 400 Hz, 115Vrms PSU).

AJJewsbury said:

Agreed - or even separated - either for one item (see BS 7671 section 413)  or several (section 418.3).

I would struggle to agree that we have met the requirements for either section.

gkenyon said:

If you take the view that this information is necessary to safely maintain the equipment, and check it continues to be safe, the perhaps Section 6 of the H&S@W etc Act 1974 requires it to be provided. Check out 6(1A)(c).

Agreed - we will make the appropriate enquiries and seek to make rectifications as soon as possible.

Given the above, I am concluding that we should be implementing ADS to mitigate the risk of electric shock presently.

Many thanks,

Richard 

ADS is assumed because the 400 Hz power supply feeds multiple items of class I equipment and the exposed conductive parts of these items are not isolated from the exposed conductive parts of other circuits and also have fortuitous connections back to facility PE via the local metalwork which is connected to the MET.

Could still be IT? So while still classed as ADS, there's no hazard from a first fault to Earth (the system simply degrades to TN) and consequently no requirement to disconnect on 1st fault. If the PSU can only supply a limited current, then there might not be a need to disconnect promptly even on a 2nd fault - if the demands of reg 419 are satisfied (i.e. the output voltage collapses to something safe - e.g. <50V).

   - Andy,

AJJewsbury said:

Could still be IT? So while still classed as ADS, there's no hazard from a first fault to Earth (the system simply degrades to TN) and consequently no requirement to disconnect on 1st fault. I

BUT ... if this is two-phase or three-phase IT system, BS 7671 strongly recommends against distribution of the neutral. (See NOTE 1 to Regulation 411.6.1. NOTE 2 gives a clue to one of the reasons why.)

So, if it is IT, with a distributed neutral, there would have to be some justification (and clear information) for its safe use and maintenance ... so again, all down to lack of information from the supplier.