UPS, RCD, RCM, Earth rods and regulations?

[ericmark]

UPS and the regs
1) Whole idea of a UPS is to be uninterruptible so fitting a RCD on the output seems to defeat the purpose. I would consider if it uses a socket outlet under 20A then 411.3.3 applies and 13A sockets should have RCD’s unless specific labelled or otherwise suitably identified socket-outlet provided for connection of a particular item of equipment.
2) Earth connection I would consider that a PEN conductor should never be used with a UPS as if the supply cable is cut it could lose the earth connection without losing the supply. 418.3.5 requires an earth.
3) Since it is designed to be uninterruptible should RCM’s be used?
4) 430.3 says there must be a protective device but where the design is so that overload will cause the unit to crowbar into a safe state is this enough?
5) 433.3.3 does refer to conditions where overloads may be omitted. But the list doe not include servers etc. May be if powering telephones than these may be considered safety equipment.

My own thoughts are where there is a single output from a UPS then like with the bathroom transformer there is no real danger as if the line was to connect to earth then it would still not give anyone a shock.
However where the UPS has multi outputs supplying more than one item then it should have a RCD and it’s own independent earth system which does not rely on the supply authorities earth.

I also would consider an earth rod is an extraneous-conductive-part when the supply authorities earth is connected and can be connected at all times.

The large servers are not really a problem having no RCD (in own locked room) and only the provision of independent earth may be an issue but often they have that anyway and call it a clean earth.

But on the smaller scale with an UPS under the desk powering computer, printer, monitor etc. Then I can see a problem. The RCD should not really be an issue as it should never trip unless there is a fault but ensuring that the UPS earth connection is never compromised is another thing and how one can ensure they are only used with sockets where there is a permanent earth independent of supply authorities earth I don’t know? Maybe Walsall Gauge plug and sockets.

I would like to hear what is done in practice. And what other think about what we should allow? I am not sure how earthing and RCD’s should be used on this one. The question came to light while fitting a 3KVA unit on a boat where we used the hull as an earth and used only a single Class II charger with no earth connected to shore power. No earth is connected between shore and the boat. The jetty is all wood and the 16A outlets are all plastic so the only way one could get a shock is from neighbours boat if they have earth connected. BS7671 does not apply and due to size of boat neither does RCD (Recreational Craft Directive). But my son compared the setup to an UPS and that got us thinking. If we wanted to do same with a caravan what would the rules be then? And with solar panels inverters on caravans are getting more popular. I have not seen a single caravan with a earth rod connected! Even when they have a generator. Although I have seen greasy spoon cafes on side of the road with rod next to generator.

 

[ColJack]

server racks are usually supplied by 32A ceeform sockets under the false floor aren't they?
411.3.3 also says intended for use by ordinary persons and for general use..
sockets for a server is not for general use, and is not for use by an ordinary person..

 

[ericmark]

As I said the larger units I don't see as a problem with clean earths and restricted access. But the smaller units which are easy to move and supply more than one item but often only 2 or 3 as a problem. One for complying with regulations needing RCD's on 13A plugs and any RCD in distribution board will do nothing. And second ensuring the earth remains intact even in a power cut.

Plus of course the caravans where solar panels are more popular which has also resulted in more and more 12vdc to 230vac inverters. Even have one in my car and no RCD on outlet.

 

[ColJack]

if it's portable and supplies the items directly, then there's no problem with it.
it's a piece of equipment.. it's output is not hardwired so we don't have to do anything with it surely?
the majority of portable UPS's have IEC connectors on the back anyway, not 13A plugs..

 

[timtheenchanter]

even non-portable UPS have IEC, I have a unit at home, rack mount, 3KVA unit, weighs a hefty 40Kgs with IECs, likewise the brand new ones we have just installed at work are all IEC, those IIRC are 8KVA

 

[ericmark]

if it's portable and supplies the items directly, then there's no problem with it.

I would have thought any device which is designed to defeat the operation of a safety device must be of some concern. It does not really matter as to if it is plugged in or hard wired. Reading 415.1.1 there is nothing to suggest the RCD is no longer required if it is a temporary installation. After all caravans plug in and they need two RCD's one belonging to site and one belonging to caravan.

it's a piece of equipment.. it's output is not hardwired so we don't have to do anything with it surely?

Most equipment I have PATested has been of a plug in type. It still needed to be PAT tested. The change is with BS7671:2008 requirement for RCD protection. Up to 2008 the UPS only needed overload protection which was normally inbuilt in the design. However the fact that it plugs in causes more of a problem as once unplugged to re-commission (i.e. plug in to any other socket) it will need to comply with regulations in force now. In real terms it will depend on the PATesting and it could have a 4 year ticket. But at some point it will need PATesting and I do not truly know how to deal with it?

the majority of portable UPS's have IEC connectors on the back anyway, not 13A plugs..

The type of plug does not really effect anything except if the plug and socket supplying the UPS was a special ensuring the earth will remain intact during a power cut.

I just had not thought about the problems with a UPS before. When one looks at a unit like the one shown it is plainly to supply more than one item but it could contain all sorts of protective circuits inside. However in the main I have not been provided with the details and other than checking the earth connection through the unit was OK in the same way as with an isolation transformer I have done little in the past to ensure the power will turn off with overload. And I had not considered what would happen if an item of equipment plugged in was to have an earth fault.

Since many units have semi-conductor switches and run direct from mains until a power cut it would trip the feed RCD but then the UPS could be connecting the exposed-conductive-part of one item to earth and the exposed-conductive-part of another item to line due to fault in second item.

I don't know how we can protect and still ensure uninterrupted supply? If smaller and only one output may be it would be OK be even a 350 to 500VA unit is shown with 4 outlets.

 

[bernardgreen]

If the output of the UPS is floating and not referenced to earth then there is no point in bringing the mains earth through the UPS and on to the equipment fed. Only an "earth" derived from the output of the UPS would be effective in the tripping of any safety devices on the output of the UPS.

As regards safety to the users of equipment fed from the UPS then the principle of the isolated UPS is the same as the transformer backed shaver socket in the bathroom. No earth required in the shaver.

That said some UPS outputs have an "earth" at the centre tap of the output that is used to "earth" the filters in the power supply units of the equipment and this earth is often also grounded for noise reduction purposes.

 

[ericmark]

As regards safety to the users of equipment fed from the UPS then the principle of the isolated UPS is the same as the transformer backed shaver socket in the bathroom. No earth required in the shaver.

I would agree that a "Single" output UPS is same as a shaver socket If it isolates but two problems.
1) Many have multi outputs
2) Many use semi-conductor switching and do not isolate until there is a power failure.

The second I can see how there is really little problem as until the supply is removed the supply RCD will protect.

But I would want to see the

Isolation sign

• ericmark
• 12 Mar 2009
• 10

sign which it is unlikely to have if it only isolates when in power cut mode.

That said some UPS outputs have an "earth" at the centre tap of the output that is used to "earth" the filters in the power supply units of the equipment and this earth is often also grounded for noise reduction purposes.

I have in the past had problems with centre tapped earths. Most of our equipment is designed for a single phase supply not a split phase supply and often there are only fuses in the line supply and nothing in the neutral but with a split phase supply we have no neutral only Line 1 and Line 2.

Thanks for your thoughts though as I am truly uncertain on this one. And I wonder if many be there is some in built safety feature. Some it seems auto close down the PC so the time over which any fault could cause danger is very short.

However not all UPS are used with PC's and not all are purpose built units but may consist of charger, battery and inverter. I see a little is talked about when using PV systems and I have seen mains, PV and wind generators all feeding the same battery and inverter to give an uninterrupted supply from multi sources. And I am sure these should have earth rods and RCD's.

The more I look into it the less I like what I see. And details of systems do seem to be very lacking.

 

[maltaron]

There is a difficulty using RCDs with small generators and invertors. Allthough I also advocate using them and am certain they will work if a fault condition occurs it is generally impossible to use an RCD tester as a local earth rod will not provide the neural-earth connection required by the tester. The push to test on the RCD does work.

 

[ericmark]

Thank you all for your input. For anyone else reading this IEC refers to one of a set of 13 connectors controlled by International Electrotechnical Commission.

I look forward to any further comments.

I have been directed here and from what I understand with overload many UPS auto switch to direct supply so the MCB on the supply still protects the system. If the supply has failed then it still switches to direct supply on overload so it will then shut down.

However to me any system using UPS would still need earth rods which would be of course an extraneous-conductive-part until the power had failed and once failed then the system becomes TN-S and the earth rod becomes the source earth.

This does not answer the RCD question. 411.3.3 Additional protection must apply unless it comes under:-
(a) socket-outlets for use under the supervision of skilled or instructed persons, e.g. in some commercial or industrial locations, or
(b) a specific labelled or otherwise suitably identified socket-outlet provided for connection of a particular item of equipment.

Although I am sure in some cases (a) and/or (b) will be valid I would have thought that UPS adverts would have offered the option of RCD protection. And as yet I have not found any with it built in.

Although you can get plug-in RCD units they are in general larger than a plug and where there are multi outputs would likely foul each other and where one of the IEC sockets are used one is unlikely to be able to buy them in UK.

I have now tried to read many instructions for UPS and the details available on the net seem very lacking in what safety system is built in. Many tell on how they will auto instruct the PC to close down. And how the LAN is also feed via the unit but as to what they do on over load or earth faults seems to be completely missing.