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W

wattsup

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Problem with RCD on transfer
Help, advice or maybe the problem is, please!
I have a prob with one RCD tripping when on generator supply
When on utility supply all ok.
Basically I have two rcd’s protecting an auto transfer switch control.
One rcd protects when in mains utility supply mode, the other protects when in generator supply.
The transfer works fine, if I mimic a power failure, the generator cuts in, the contactors switch to generator supply, all ok.
When I restore mains power, the contactors revert back and cut out the generator.
Except that when in generator mode, the rcd for the said supply trips.
There is no fault in circuit, tested ok. If I bypass the generator rcd all works fine, but I would rather have the rcd online, if I can fathom why it’s tripping.

 
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U

uksel

  • Thread Starter Thread Starter
  • #2
could it be common neutral from the generator feed to the utility supply.

not sure how it's wired but it could be responding like a borrowed neutral = inbalance

is the transfer switch/contactor switching douple pole?
 
W

wattsup

  • Thread Starter Thread Starter
  • #3
yes dp on all contactors.
Today I bypassed rcd to check voltage L to E, getting 110v L to E, 240v L & N.
Martindale tester says no earth..
So it seems a floating earth, yet all connected as per manufacturers instruction
 
J

jimes

  • Thread Starter Thread Starter
  • #4
110v live earth?......i would say rcd is tripping as the gen supply is not all that stable, upstream inbalance can make rcd's trip.........filtering may help
 
W

wattsup

  • Thread Starter Thread Starter
  • #5
The gen supply is more stable than the grid! +/- 3v, frequency spot on

>>110v live earth?......>> rather than 230v, if using a test lamp I would guess lit dim. (similar to a poor earth, rather than none)
 
J

jimes

  • Thread Starter Thread Starter
  • #6
have you tried swapping rcd's,only 2 things that are different,rcd and supply?
 
U

uksel

  • Thread Starter Thread Starter
  • #7
if your L to N on the martin dale is when checking the 110v then this would be normal if the 110 supply is split to 55v a leg.

i think the borrowed neutral idea could still be investigated further, even if they are douple pole switches, thinking about it - the neutral will still be common, if you dont mind my saying, set the genny RCD to off and try it, then run the genny with the mains supply to off and see if you still get the nuisance tripping.

not sure if this would work for you, but all else i could possibly think of (without looking at your setup) is that the genny is producing some eddy currents which would account for your loss/inbalance - perhaps!

does the utility supply have a load connected when you are testing?
 
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M

Moog

  • Thread Starter Thread Starter
  • #8
whats the type of the RCD that is tripping? is this a delay issue? would have thought that motor RCD would be type C anyway but worth a check
 
W

wattsup

  • Thread Starter Thread Starter
  • #9
A bit more info (the suppliers have my money and do not answer emails, phone always on answer machine)
Anyhow..the prob seems to be the neutral earth link (which is advised, if existing rcd on cu to work correctly, in effect config. the gen to supply as the mains is. In this case tn-c-s.
So I link N&E downstream of my new rcd (ouput from gen, at supply point)

So now I am supplying same for same as the grid, neutral earth combined.

This method trips the main gen feed rcd I have installed. (I installed because it is a big clumsy looking metal auto transfer switch, so I reasoned if on gen supply the rcd would give added protection. If I remove the n&e link all ok, except I get 'no earth' or a very poor earth. ( I don't know why, all earths including spikes are combined)

Of course the auto transfer also controls mains grid power, I installed an rcd to protect this part of the installation, that works fine, no problems on mains power, rcd ok earthing ok.

So to sum up, I have two seperate feeds to one transfer switch (both cannot operate at the same time, protected mechanically and electrically) On mains ok, on generator, trip, unless I remove the n&e link at the gen supply, then I get no earth?

I'm thinking I have to remove the rcd protecting the gen feed and replace with an ordinary dp isolator, and then replace neutral earth link from the gen. I assume this will give the real earth and existing rcd in the cu will work correctly, however if I do this the auto transfer switch will not be protected with rcd if under gen power, its not what I expected. I have not come across this scenario before. I know the gen has a floating earth so I must really provide the n e link, to give cover for existing rcd's. Except the bit I did is not covered by rcd, yet the regs say it must be

I knew there was a problem straight away, an outside light. When I installed the rcd for mains protection; switch the outside light on, trip. (the light was on the unprotected side of existing cu) No problem the fitting was full of damp leaves gunge etc, yet on generator feed no trip...Oh no I thought!, no n e link no rcd work, install n e link, trip straight away even if nothing switched on... I may have a problem huston

Can I leave a metal box outside, protected within an ip rated cabinet (similar to a meter cabinet, not sure of ip, 66 I think, without checking rating, but weatherproof
Without rcd protection? The problem is, there is a plug-in 12-core to the unit, control feeds etc. So how is it classed?
I'm only trying to protect one metal box, which really is not used by the customer, they should not go near it, unless of course a trip occurs then it will need resetting
 
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D

dixon9

  • Thread Starter Thread Starter
  • #10
whats the type of the RCD that is tripping? is this a delay issue? would have thought that motor RCD would be type C anyway but worth a check
This looks favourite imo. What do the manufacturers instructions on the genny installation say about type of RCD? Type S?

During the switch over there could be a delay issue for the RCD which is too sensitive for this application?

Also came across this:

Residual current devices on portable generators
 
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U

uksel

  • Thread Starter Thread Starter
  • #11
earth is earth, you should be connected continuously throughout the circuit. so the earth of the metal should be directly connected to the earth of the mains distribution board, if there is any earth fault it should trip while on mains. if you have neutral and earth linked then it's obviously going to trip while running off the genny (if the switch is double pole) if the switch was single pole i imagine the mains RCD would trip continuously

as another thought - if you are looking to have the supply being uninterruptable, would you really want to risk nuisance tripping resulting in no supply whatsoever when you need it most.

your emergency supply should be bomb proof, or i imagine that is what the client would want?

IP66 is required for weatherproof/outdoors although i believe IP65 will suffice, metal casing is fine withour installed RCD, but you will need a good earth reading to ensure your tripping times are still within the regs at the mains supply.
 
W

wattsup

  • Thread Starter Thread Starter
  • #12
>>if you have neutral and earth linked then it's obviously going to trip while running off the genny<<<
Sorry I don't get that, the link is on the supply point, not distribution side of rcd.

>>as another thought - if you are looking to have the supply being uninterruptable, would you really want to risk nuisance tripping resulting in no supply whatsoever when you need it most.<<

Nope I don't want nuisance tripping, nor will the client. But I don't write the regs.

Just had a thought-When I fake a power cut by throwing the breaker, the earth neutral will be broken -tncs system...no neutral = no earth? but of course the earth is not broken, the main earth still goes back to utility cut-out (n & e combined)So the genny takes over and is now relying on the earth spike, rather than the linked neutral earth from mains supply. When in genny mode and test at a socket in the property, I get 230v L and N, but only 110v L & E.So I presume the genny neutral is not a 'real neutral' 2 x 110v legs?Is it possible the earth spike is a poor grounding (btw its ****ed it down all week, but maybe the soil condition is 'rocky' or whatever?All earthing; cu, generator frame / main earth / earth spike, transfer box is combined, all wired in 16mm.I just don't get the 'bad earth' when on generator supply.

Perhaps the earth reading is misleading, if both legs supply 110v (I need to check neutral to earth also, but at the time I overlooked..I will do so on Monday...anyhow if the line is only 110v then actually 110v line earth is good, not poor.
Still don't explain the rcd probs with n e link though.

I have installed many big stuff generators before and have not come across this before, but this is a tiny (comparitively) 19kva unit, perhaps previous suppliers pre-configured the set to suit.

I will not use these suppliers again, after sales is very very poor
 
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U

uksel

  • Thread Starter Thread Starter
  • #13
>>if you have neutral and earth linked then it's obviously going to trip while running off the genny<<<
Sorry I don't get that, the link is on the supply point, not distribution side of rcd.
my apologies, i thought you were saying that there was a neutral earth link in the transfer switch, for some reason...

Nope I don't want nuisance tripping, nor will the client. But I don't write the regs.
what are the circumstances demanding that you have an RCD on the generator side?

If this is a PTO generator, most of these types of generators are dual voltage, 110V and 230V supplied by a split-phase winding, In theory you should be getting 230V between L and N and between L-E and N-E 115V but this is not always the case, these generator are a pain as the generator relay keeps dropping out a lot which will affect electronic equipment, any RCD's should be at the panel as the generator has its own protection, you need to find out about the generator, what is the make/model??
 
W

wattsup

  • Thread Starter Thread Starter
  • #14
Hi uksel, maybe I'm not reading the regs correctly..but as I understand it, all outside residential electrical equipment must be protected by rcd. The generator is outside, the transfer switch is outside. I cannot trip the generator itself, that is inbuilt, but I ought to be able to protect any equipment it supplies, in this case the auto transfer switch. Also the existing rcd on the users cu will not function correctly if no refference to earth when on generator supply.

That I know is a regulation, the rcd should operate as normal, no matter where the supply comes from. Like I said no problem on mains supply, but on generator (manufacturers reccomend if used for back up, an e n link should be installed from gen output, usually on the commando plug. When I do as instructed, the genny feed protected rcd trips...no probs with mains..
 
U

uksel

  • Thread Starter Thread Starter
  • #15
chapter 55, section 1 of the 16th edition regs: Generating Sets

Additional requirements for installations where the generating set provides a switched alternative supply to the distributors network (standby systems).
551-04-03: Protection by automatic disconnection of supply shall not rely upon the connection to the earthed point of the distributor's network when the generator is operating as a switched alternative to a TN system. A suitable earth electrode shall be provided.

Additional requirements for installations where the generating set provides a supply as a switched alternative supply to the distributors network (standby systems).
551-06-02: For TNS systems RCD's shall be positioned to avoid incorrect operation owing to the possibility of a parallel neutral path.

a few things i would point out

- yes, it is the 16th regs - i haven't had the refresher course or bought myself an up to date copy of bs7671
- I believe you said that the earthing system is TNC-s not TNS, but i think it may still apply as your neutral is still not isolated because of the earth connections.

i would gauge an opinion here wether an RCD at the CU is acceptable as means of protection, i think it will be. the generator is outside although it is not a circuit to be protected by a permanent supply, run the supply cables in earther metal conduit to your one way consumer unit and put an overcurrent protection device in there, not an RCD, leave the RCD protection for the clients main consumer unit

would anybody else say this was acceptable or not? perhaps reasons/regulations why also would be nice

Hi uksel, maybe I'm not reading the regs correctly..but as I understand it, all outside residential electrical equipment must be protected by rcd.
regulations i believe in this instance state that RCD's shall be used to offer protection for circuits with the provision to supply portable equipment outdoors.

key word - portable

if this was the regulation you were referring to, it applies to socket outlets. in certain circumstances even the use of over-current devices may be omitted for standy systems :eek: (although i do not think this would apply to your situation)
 
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