Rectivertors

[Steebs]

Hi Guys,

This is a bit of a long post, as I'd like to detail everything, and take input from others.

Let me start by saying that I did my electrical apprenticeship in Australia, completed all additional courses to become a Licensed A Grade Electricial, as well as a Registered Electrical Contractor. These are required there to be self employed. The wiring rules as a whole are virtually identical, with a few differences, particularly regarding earthing, and the use of Ring Circuits.

When I returned to the UK, I immediately booked in for 17th Edition, and recently did 18th Edition. These to familiarize myself with local requirements. So on the whole, I consider myself a sufficiently trained in these areas.

So, on to the dilemma I have.

I have been asked to wire up a supply to a Wireless Internet Communications cabinet. Generally this is easy, as its nothing more that a standard install. Where this differs is the customer wants to use Rectivertors. For those unaware, they are power supplies, charging batteries and an invertor taking power back to mains voltage. In other words a very fancy UPS.

Again, a standard installation, however the complexities come from the fact that the customer wishes to use a generator and renewables as methods of charging. The rectivertor has multiple modules, each which as far as I am able to determne has power inputs isolated from any other. Each would, however share the chassis ground. Now the generator needs to be earthed and I imagine the generator neutral bonded so that any protective devices will trip should a fault develop.

Now to add the third level of complexity. The output is effectively a segregated supply, but at 240V level. Ground and neutral are isolated from one another, and it is this supply which will be supplying teh electronics used by the communications equipment. This equipment includes network switches, routers and power supplies for radio devices.

So, to recap, I technically have three seperate supplies, each with its own earth and neutral. My understanding is that to prevent nuisance tripping, the neutral from the mains and the neutral from the generator must be isolated from one another. That wouldn't be a problem, but if earths are then bonded together so that RCD's will operate correctly, then neutral being bonded to earths are also connected.

I have been through 551 of the regs over and over again, and I still cannot see anything definitive. If I was in Australia the answer would be simply a matter of sinking three earth stakes, one for the incoming mains, one for the generator and one for the segregated output. You see in Australia a DNO earth is not provided, therefore every property sinks their own earth rod, and bonds the neutral to that earth at the main switchboard, at a point called the MEN ( Multiple Earthed Neutral, multiple because every property bonds the neutral which would be common, to earth).

I'd like input of how you think I should approach this. The Aussie option seems the best one, and provided that this complies with 551, then I can go ahead and install it. Obviously no earth stake at the DNO supply.

I look forward to hearing from any and all.

Thanks in advance

Steve

 

[Lucien Nunes]

My understanding is that to prevent nuisance tripping, the neutral from the mains and the neutral from the generator must be isolated from one another. That wouldn't be a problem, but if earths are then bonded together so that RCD's will operate correctly, then neutral being bonded to earths are also connected.

There is no problem with two independent supplies each having their neutral earthed, and therefore being incidentally connected together via earth. In theory all systems with an earthed neutral are ultimately connected together via earth, and provided each system's neutral is earthed only at one point upstream of any residual current protection, no circulating currents can flow that would cause nuisance tripping.

The problem would be if the neutrals themselves were connected at another point, creating two parallel paths for the neutral current, one of which bypasses the residual current protection.

 

[plugsandsparks]

When you say 3 supplies, is that mains from DNO, client solar panels (on-grid) and a generator ?

 

[Steebs]

When you say 3 supplies, is that mains from DNO, client solar panels (on-grid) and a generator ?

I counted the segregated invertor output which would actually be supplying the equipment as the third supply. Solar will directly charge the battery array and is discounted for the sake of simplicity. Perhaps this third segregated supply is of most concern. Leave the neutral floating and use elcb or bond and use rcd?

 

[Strima]

Might be easier to run everything through a standard UPS then rectify to DC if required for some kit?

I do occasionally see rectiverters on site but we try and avoid using them due to power factor efficiency.

 

[pc1966]

A quick look for "rectiverter" shows it is basically an on-line (no break) UPS!

True, the efficiency is a couple of percent less than the more conventional ones but we have generally preferred them as you really don't get any risk of crappy machines rebooting on a glitch. For some odd reason, we have seen conventional UPS (OK, cheapish ones on the 1kVA range) cause PCs to reboot on a supply glitch that did not reboot those PCs direct of mains!

The issue is really one of not being permitted to link the N & E within an installation, that it has to be done in one place and before the distribution board.

Will the rectiverter system have isolation between the AC inputs and the DC bus? If so not really a problem.

Otherwise can you simply use a transfer switch to swap between the mains and the generator? That way you don't end up with two N-E links and one beyond the DNO supply boundary but linking the DNO supply.

I think the solar PV is essentially isolated anyway as the panels are floating (and usually have an impedance monitoring system to shut down if not).

 

[Steebs]

A quick look for "rectiverter" shows it is basically an on-line (no break) UPS!

True, the efficiency is a couple of percent less than the more conventional ones but we have generally preferred them as you really don't get any risk of crappy machines rebooting on a glitch. For some odd reason, we have seen conventional UPS (OK, cheapish ones on the 1kVA range) cause PCs to reboot on a supply glitch that did not reboot those PCs direct of mains!

The issue is really one of not being permitted to link the N & E within an installation, that it has to be done in one place and before the distribution board.

Will the rectiverter system have isolation between the AC inputs and the DC bus? If so not really a problem.

Otherwise can you simply use a transfer switch to swap between the mains and the generator? That way you don't end up with two N-E links and one beyond the DNO supply boundary but linking the DNO supply.

I think the solar PV is essentially isolated anyway as the panels are floating (and usually have an impedance monitoring system to shut down if not).

Thanks for your comments, and much tends to be along the lines of my thinking. I find in strange circumstances such as thesem its often great value hearing from others. You are quite correct, in that a rectivertor is just a fancy UPS. To set the record straight the other type of backup supply is properly termed an SPS ( Standby Power Supply and it is not truly Uninterruptable)

We are in contact with the manufacturer of these devices in order to obtain as much information as possible.

The sticking points for me are still the output supply, which has isolated E&N and is segregated from the incoming supply.

If we forget for a moment that there is a big battery bank in the middle, and look at this installation as a big isolaton transformer. Lets say for argument sake that the incoming supply and the generator supply are housed in a seperate box. All that connect these two boxes together is a pair of wires. The second box contains the gubbins to make a 240V supply once again. This segregated supply will have its own Active and Neutral (Hence my terminology of a thirds supply) and this active and neutral feeds the equipment. For protection, it should have some form of earth leakage detection and shold also have overcurrent protection, and so it becomes a supply in its own right.

Sec 551 of the regs doesn't seem to tell me that I should treat this as a seperate, isolated supply, as the way I read it, much makes the assumption that the supply is provided by a backup generator or the DNO.

I'll continue to read up and if I find anything myself I will post back here in case anyone else comes across a similar situation. I hope you don't and hope your clients listen and stick to a well known KISS pronciple.

Cheers

Steve

 

[pc1966]

If the rectivator output is isolated at all times that is rather different from most UPS, where the output is normally reference to the supply neutral (though I think some isolate when the supply fails, not sure what that mans for ADS of course as having an IT supply is not for general use!).

In that case it would be like an isolating transformer and then you would link N&E at the rectivator so you have essentially a TN-S supply from it.

 

[pc1966]

The way I read the rules:

• Thou shalt not use TN-C cabling as t'is the Devils' work
• The number of thy E-N links shall be one. Not none, not two, nor any other number, as they art abominations!

 

[Steebs]

If the rectivator output is isolated at all times that is rather different from most UPS, where the output is normally reference to the supply neutral (though I think some isolate when the supply fails, not sure what that mans for ADS of course as having an IT supply is not for general use!).

In that case it would be like an isolating transformer and then you would link N&E at the rectivator so you have essentially a TN-S supply from it.

Thank you. I think I mentioned in my initial post, back in Oz, where we have two seperate and simple earthing systems, it would be job done, but the extras here in the UK have always been a source of confusion for me. My goodness, what happened to KISS?? Don;t get me started of cable colours!! LOL

The way I read the rules:

• Thou shalt not use TN-C cabling as t'is the Devils' work
• The number of thy E-N links shall be one, not none, not two, nor any other number, as they art abominations!

Back in Oz, I would just bond each N to E and sink individual earth rods, as remember the DNO doesn't supply an earth there. Oh how simple life was!!

If these people had only listened to me, and stuck in as ATS and a bog standard UPS I wouldn't be up all night reading regs and pulling my hair out. This is what happens when nerds meet the real world I guess.

 

[DPG]

Have you checked appendix 9 of the regs? (page 497 BBB). Not sure it has the info you are looking for but may help.