Navitron Renewable Energy and Sustainability Forum

Energy/Electricity Storage and Use/Grid Connection => Off-Grid, Batteries & Inverters => Topic started by: AndyB on April 26, 2009, 07:36:07 PM



Title: Neutral isolation advise
Post by: AndyB on April 26, 2009, 07:36:07 PM
Hi, at the moment i use a relay that has the coil connected to the inverter and switches certain circuits back over to mains should i run out of battery charge.

Do I have to keep the inverter Neutral and the house supply Neutral separate or can they be common-ed. What is best practice and why?

Can anybody point me to the relevent regs regarding this sort of switch over i suppose its like supplying a house with a standby generator.

Cheers

Andy


Title: Re: Neutral isolation advise
Post by: daftlad on April 26, 2009, 08:24:03 PM
Just had a quick peek in the regs and the contactor for isolation should be to BS EN 60947-4-1 provided it is also marked with the symbol for isolation, contactors to BS EN 61095 are not permitted.
As far as commoning the neutral is concerned it is permitted under some circumstances BUT there are possible problems, I would seperate them.
hope this helps
laters


Title: Re: Neutral isolation advise
Post by: AndyB on April 26, 2009, 08:57:18 PM
Fantastic, thanks. Can you enlighten me on the possible problems of joint neutrals

Andy


Title: Re: Neutral isolation advise
Post by: tony. on April 26, 2009, 09:03:25 PM
the neutral is considered a line conductor so there could be a few.

lack of true isolation between circuits.

backfeeding voltages etc

as daftlad says, it would be best all round to keep them seperate.

changeover switches i have the 63amp version from tlc, 63 amp if fine for me as it only feed an essential board in the event of REC/DNO mains failure.

regards

tony


Title: Re: Neutral isolation advise
Post by: daftlad on April 26, 2009, 09:05:45 PM
sumut to do with lightening and harmonics, I would just split them out of principle.
Is it a problem to split them? I guess it may mean a new consumer unit or a retro fit second neutral bar.
Don't rely on the electricity board earth because if there is a power cut the earth could also be broken and leave you earthless (is that a word?) knock in a spike and put everything on an rcd.
laters
ps i will have another look at the neutral thing later.


Title: Re: Neutral isolation advise
Post by: tony. on April 26, 2009, 09:19:26 PM
watch out about sticking in earth rods, check your earthing arrangement first,


for example say you live in a small street with a tncs supply and you have fitted your own earth rod, in the event of the DNO loosing their neutral, your rod will be carrying the fault current for the whole street, ok that worst case senario, but could happen.

tony


Title: Re: Neutral isolation advise
Post by: daftlad on April 26, 2009, 09:34:12 PM
Tony
I see your point BUT if that happens a TNCS system is also a PME which means there is multiple earths provided by the electric board.
Also it could be worse, if the leccy board neutral/ earth failed all the bonded metalwork would become live which would make having a bath a bit of a buzz.
What does DNO stand for.
laters
ps I think i actually read about putting a rod in for TNCS systems under certain circumstances which for the life of me i can not remember
pps Tony is right if you dont really understand earth paths, neutralising points, rcd's, you do need to be careful


Title: Re: Neutral isolation advise
Post by: Ted on April 26, 2009, 09:43:06 PM
The first point in this decision process should be to determine the type of main supply earthing - usually TN-S, TN-C-S or TT. But in pretty much any instance you would want to keep your neutral separate from the neutral of the supply as others have said.  Reg 551.4.3.2.1 requires that you have separate earthing if you haven't already got TT.  In fact if you have a copy of the Regs then I suggest you read all of Section 551.


Title: Re: Neutral isolation advise
Post by: tony. on April 26, 2009, 09:48:21 PM
daftlad,   in a tncs situation you would possibly use a earth rod for a outbuilding/ garage that is outside the equipotential zone, so you dont export the supply authority earth, you fit a rod or two,assuming there is exposed metalwork,

so swa from house properly earthed etc, connected into a plastic enclosure swa earth not then connected to anything else, or a insulated gland adaptor could be used instead of a plastic box, but a bit more expensive.

dno. district network operator

cheers


Title: Re: Neutral isolation advise
Post by: daftlad on April 26, 2009, 10:00:23 PM
Tony
I know the situation you describe, agricultural outbuilding where TNCS is no longer permitted or a petrol station, or in an agricultural situation where cross bonding is not possible.
I am sure it referred to a situation more like IT or domestic where the earths were connected together.
Andy
Get down the library and photocopy the relevent pages of the regs, i will post the page numbers that i looked at tomorrow.
laters


Title: Re: Neutral isolation advise
Post by: daftlad on April 27, 2009, 10:48:06 AM
The regs to look at are
reg 537 There is a table showing what is acceptable for use as an isolator, on page 117
reg 551 Only 5 pages all about generators etc, on page 137
reg 712 this one is the special location for PV, on page 200
so get down the library and photocopy these pages.
hope this helps.... be careful
laters


Title: Re: Neutral isolation advise
Post by: AndyB on April 27, 2009, 06:58:33 PM
Wow thanks guys, the reason i ask is simple i haven a em21 energy meter (http://www.cse-distributors.co.uk/controlgear/energy-meter/EM21-72D-energy-meter.pdf) and i wanted to use one phase to monitor my inverter Kw and another to measure the normal supply but all the example circuits need a common neutral. Maybe i will have to get a second one.

Thanks


Title: Re: Neutral isolation advise
Post by: daftlad on April 27, 2009, 10:19:13 PM
Andy
I just had a quick peek at the data sheets and it occurs to me that you may be able to do it.
There seems to be current monitoring and voltage monitoring (to give wattage)
i would suggest that you only monitor the voltage of one supply but measure the current of both.
This would give accurate wattage of one supply but the other would not be so accurate because the voltage would be slightly different.
You would have to talk to the manufacturers or someone who knows this kit better than me but it might work?
laters


Title: Re: Neutral isolation advise
Post by: EccentricAnomaly on April 29, 2009, 08:49:51 PM
This would give accurate wattage of one supply but the other would not be so accurate because the voltage would be slightly different.

Wouldn't there also be a problem with the phase/power factor?


Title: Re: Neutral isolation advise
Post by: daftlad on April 29, 2009, 09:17:43 PM
Maybe/ Probably?
Thats why i suggested speaking to the manufacturer
Maybe it will only cope with 2 phase or 3 phase, even if the neutrals were linked the frequencies would be all wrong.
Best to ask but don't try anything until the manufacturers have given the go ahead
laters