RCD's in UPS distribution

[raithrovers1]

Hello Gents
We have just replaced a 120kVA UPS on an oil & gas platform with a newer identical UPS. The original had 34 critical supplies fed from a distribution which all had 300mA RCD's on them.
The new system had the same 34 MCB's but a were supplied with 30mA RCD's by mistake!! this was not picked up at FAT so we have what we have at the moment!
Anyway, the way I see it is that critical UPS supplies are not normally fed from an RCD circuit whether it is 30mA (personal protection) or 300mA (fire/ equipment protection).
We are trying to justify the removal of all of the RCD's and just leave the correctly rated MCB's. Do you guys have any comments on why this would not be advisable?
Thanks as always for your input.

UPS engineer

http://www.calibrepower.co.uk

 

[raithrovers1]

I have been doing more investigations and found the following:
According to UK 17th edition 422 precaution where particular risk of fire exist:
A 300mA RCD is required in locations with fire risk unless mineral insualted cables are used. This is due to 300mA limiting the potential wattage of an isulation fault to less than 60W at 240V. This is classed as the minimum wattage at which fire can start in combustible materials such as dust and fibres.

Unfortunately these regualtions do not apply offshore so can not be referred to. Also it is quite obvious that as some of my load cables may go through modules that have the potential of gas ingress I would say that 60W is more than enough to ignite a flammable atmosphere. Therefore are the RCD's serving any pratical purpose?

UPS engineer

http://www.calibrepower.co.uk

 

[ScottyUK]

Can you meet the required disconnect times without the RCD?

Have you checked the requirements of the codes the platform was built to? Some of the offshore codes are quite different to onshore. If it's a North Sea platform then the NORSOK standards might well have been used as a design basis. I'm not sufficiently familiar to comment further.


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If we learn from our mistakes I'm getting a great education!

 

[raithrovers1]

Thanks for the reply Scotty.

It is near impossible to get the codes that the platform was built to. The platform changed hands from an oil major to a small company and the handover was very unprofessional with regards to handing over marked up drawings and technical information. Surprising but true!
Overcurrent protection is not a concern as the MCB's in the panel are all small enough for the UPS to clear a short.

UPS engineer

http://www.calibrepower.co.uk

 

[ScottyUK]

Unsurprising and true!

I wasn't meaning the UPS ability to clear an MCB on short circuit, but the disconnect time for a fault at the remote end of a cable. Onshore fixed installations require a 5 second disconnect time, sockets for portable equipment require 400ms to BS 7671. There must be equivalent requirements offshore, but I don't know what the numbers might be.


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If we learn from our mistakes I'm getting a great education!

 

[MikiBg]

Agreed with Scotty. Conditions for effective protection from indirect touch will give you the answer are RCDs necessary or not. It depends on earthing arrangement and fault loop impedance as well. Here in continental Europe IEC standards are predominant. For example, in TN systems (protective conductor from exposed parts back to earthed neutral) disconnect times required are the same as Scotty mentioned by BS. In TT systems (exposed parts earthed directly) for lower tripping current of a protective device, higher earthing resistance is allowed, so you will satisfy it more easily with RCD applied. Try to find something similar for your case.

 

[raithrovers1]

Scotty
I have just come back to this as I don't feel I have a full understanding of the situation yet.
The technical authority has come back to us and has stated that it is as you have stated. He thinks the RCD's are installed for to meet the breaking time standards.
What I do not understand is how a live to neutral fault would cause the RCD to trip.
I realise that a live or neutral fault to earth would trip within the times stated in the standard (5 seconds).
Am I missing something quite obvious?

UPS engineer

http://www.calibrepower.co.uk

 

[ScottyUK]

You're absolutely right, an RCD won't trip for a line-neutral fault. The correct device would be an RCBO, or the RCD would need to be used in combination with an MCB. An RCD on its own offers no overcurrent protection to the circuit - surely that isn't how this installation has been constructed?


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If we learn from our mistakes I'm getting a great education!

 

[raithrovers1]

Scotty

Sorry if I confused things - the system does have RCBO's.
I just don't understand the design principle for using RCBO's in a UPS supply in an offshore industrial setting. Most, if not nearly all platforms I have been to do not have RCBO's in the UPS distribution which would suggest that the standards do not require this protection. Added to this the inconvenience of spurious tripping possibilities on critical supplies.
I am maybe looking into to this a bit too much and the system I am referring to might be a one off but I like to find reasons for engineering design so I can learn for future projects.

Cheers

Craig

UPS engineer

http://www.calibrepower.co.uk

 

[raithrovers1]

As an add on to above quote:
"As a general principle, it may be considered that socket-outlets in commercial and industrial premises needing to have additional protection by means of an RCD include the following: those in common, circulation and public areas; those in self-catering areas; those intended for use by cleaners; and those that may reasonably be used to supply mobile equipment for use outdoors."
This is taken from the electrical counsels website.
Also:
"An exception is permitted for socket-outlets under the supervision of a skilled or instructed person e.g in some commercial or industrial locations." 17th edition 411.3.3

I would therefore conclude that as nearly all of the loads fed from this UPS do not come under the above areas eg. AC lube oil pumps, control panels etc. that RCD's are not required.

Any comments?



UPS engineer

http://www.calibrepower.co.uk

 

[ScottyUK]

Hi Craig,

Your most recent post relates principally to personnel protection, but the requirements for meeting fault disconnection times would still stand regardless of whether there is a need for personnel protection. Normally you can use a 100mA or 300mA type for this purpose which gives a reasonable level of immunity from false tripping, but is useless for personnel protection. Perhaps someone was over-cautious with the design and introduced a different problem by using an RCBO?

I am slightly surprised that a welded steel structure causes problems for disconnection times if they are the same as onshore. I'm loathe to speculate too much because I'm not familiar with offshore requirements.


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If we learn from our mistakes I'm getting a great education!