High impedance readings

grousedogtom

Hi
I wired a few extra sockets in a house today and when i was finished i plugged in tester to test fault loop impedance on new circuit and discovered very poor readings up around 55 ohms PFC of 4 amps, i then checked FLI at consumer unit and got similar readings , i noticed the phase and neutral going into the esb meter and the phase and neutral out of meter into consumer unit ,the only incoming earth i could see was from the earth rod.My question is , is it up to the esb to make this a tncs system by putting in a neutralising link as the meter is sealed and why would this not have been done before as with FLI readings that high a fuse would never blow in a fault situation.

M cebee

ya fuses won't blow

you need rcd for automatic disconnection on final circuit here-which you've prob fitted

don't think esbn will upgrade without cert and main bonding etc


neutralizing is done at the cut-out

sparcocars

The setup you describe at the meter cabinet is correct. The esb only provide a phase and neutral to the customer. The neutral is earthed in the cutout with the customer earth but it will also be earthed out in the network. Depending on your location if your in an estate then it is earthed at the minipillar and if your rural and fed from overhead network then it is earthed at the first pole from the transformer and then every couple of poles. High loop impedence values don't necessarily mean a poorly neutralised system. It can also be the service cable feeding the house is too small and will need to be upgraded for a bigger one. Ring esb networks and get them to investigate. Can you give me any more info about how the power is provided to the house i.e : estate, rural and rough distance from minipillar or overhead transformer?

M cebee

it's not neutralized

prob an old rural setup

small service cable wont cause that

sparcocars

I beg to differ with the size of the service cable. I work for esb networks and if the service cable is the usual 25/16 or 35/25 and its a long run from the network to the installation the those service cables can cause a high loop impedence. A larger service cable like 70/50 or 120/70 may need to be used.

grousedogtom

The system is not neutralized at present, the esb meter and cut out is indoors beside the consumer unit , its in a rural area and fed overhead from esb pole straight into house . I presume this is a TT system as the customer earth is not connected to any part of neutral on esb cut out.

M cebee

make sure you've 30mA on your additional sockets for fault disconnection

after that upgrade is down to main bonding +cert etc

and ESBN deciding

M cebee

sparcocars wrote: »

I beg to differ with the size of the service cable. I work for esb networks and if the service cable is the usual 25/16 or 35/25 and its a long run from the network to the installation the those service cables can cause a high loop impedence. A larger service cable like 70/50 or 120/70 may need to be used.

i know what you're saying on the network and service cable -it can affect loop impedance

-to a lesser extent

Bruthal

sparcocars wrote: »

I beg to differ with the size of the service cable.

I would have to agree with Mc cebee here, the 55 ohm loop impedence is the path through the earth rod to the next neutralized position or positions, and is the main reason neutralising is used. No way 55 ohms is purely because of the small supply cables. If that was the case, switch on a 25 ohm(2kw) kettle and nothing else in the house and you now have about 80 volts on the kettle and 160 dropped in the supply cable. Even if the supply cable is 20 ohms, you will have 130v on the kettle, nothing can argue with that.

The supply impedence which is really whats being measured on a neutralised setup loop test (as well as the earth and live in tested circuit)will have to be down close to 1 or 2 ohms at least, as even 10 ohms would have a noticable affect on items in the house.

Loop impedence is through a path from the live into house, onto the earth, and back out on the earth and eventually into the neutral. On a neutralised system it simply comes in on Live, back to earth bar of CU, and straight to neutral at meter.

In a non neutralised setup it goes back to earth bar, but now the only path is down the earth rod(+earthed pipes), and a single earth rod can have a high impedence. And thats likely the cause here.

A small service cable will of course have some bearing, but only to an ohm or 2, not 55 ohms. And if you get 55 ohms on a neutralised system, then there is a serious supply problem assuming the connections in the house are ok.

I work for esb networks and if the service cable is the usual 25/16 or 35/25 and its a long run from the network to the installation the those service cables can cause a high loop impedence. A larger service cable like 70/50 or 120/70 may need to be used

It will never be up at 55 ohms on a neutralised installation unless the run is several kilometers. 100 meters of 2.5 copper cable has an impedence of 1 ohm, so a supply fed in in 100 meters of 2.5 T&E would have an impedence of 2 ohms.

The problem in the case here is the earth rod, and as i argued before, a single earth rod can have a very poor connection with the ground, some said no its a great connection, but its not in reality, especially when the ground is dry. Neutralise it and it will likely be down close to what it should be, and if it still fails to be below the minimum, then the supply cables might need replacing, but it will only be 1 or 2 ohms above it if at all, not anything like 55 ohms.

M cebee

55ohm is fine on a standard domestic TT(when used in UK etc.)

if that's excluding parallel paths at the MET such as buried pipes etc


you measure your external fault loop impedance on TT through the 'earthing conductor'(only) for test purposes


you have to have rcd though for ADS(automatic disconnection)

M cebee

simply neutralizing an installation like this -would be hazardous

you'd have voltages all over the place in the event of a supply fault-without 'main equip bonding'

and the existing network may not be suitable

Bruthal

M cebee wrote: »

55ohm is fine on a standard domestic TT(when used in UK etc.)

I was not saying its not fine for TT, i was saying, in agreement with yourself that it is not the supply cables likely causing the results, but the fact it is a TT.

M cebee wrote: »

simply neutralizing an installation like this -would be hazardous

While i said neutralise it and it will bring the loop down, i did not actually mean for him to neutralise it himself without taking the proper steps, but that if it was neutralised, the impedence would be likely down to single figures, regardless of the supply cabling size.
A badly worded post by me overall, not uncommon where im involved.

If its staying TT it needs a main 300ma time delayed RCD probably.

M cebee

i wasn't referring to your post at all:)

Bruthal

M cebee wrote: »

i wasn't referring to your post at all:)

I am beginning to think everyone is right, i am nuts:eek:

grousedogtom

OK thanks for all yer helpful replies, i spoke with the esb today and they seemed surprised that it was still a tt system and were very anxious to change it to tn cs asap. What exactly do i need to do before this is done , bond all copper in hot press with 10 square , bond sinks etc??. Also can someone explain how exactly by changing to a tn cs that a fault on esb side like a broken neutral would be so dangerous , many thanks

Bruthal

grousedogtom wrote: »

Also can someone explain how exactly by changing to a tn cs that a fault on esb side like a broken neutral would be so dangerous , many thanks

Because the earth bar in the MCB board is connected to neutral in the tn-cs system, if the neutral breaks outside the neutralising point, the neutral, and now the earth bar, along with everything connected to it is now an open neutral, at 230v, with only the earth rod connecting it to ground, but in practice, the single earth rod at a house does little to keep the open neutral much below the 230v.

Because the earth bar is now directly connected to neutral in the neutralised installation, all earthed items will now be at the open neutral potential. Thats why everything in a neutralised installation must be bonded, so at least it will all be at the same potential.

If absolutely nothing was on in the house, the open neutral would have nothing on it. Not realistic though. last time i seen it happen, it was the main neutral onto a mini-pillar that failed, and so a group of houses had in effect an unbalanced 3 phase supply between them with no neutral. One house i got call from had a fair bit over 300v in it L-N.

grousedogtom

Ok i get it now, so if i connect all usual pipework etc the same as a new installation to earth its all safe then to change to tncs, yes?

Bruthal

grousedogtom wrote: »

Ok i get it now, so if i connect all usual pipework etc the same as a new installation to earth its all safe then to change to tncs, yes?

You will have to get onto ESB networks then, and see what they say, cant really just neutralise it yourself, although you would actually be connecting it yourslef.

M cebee

i wouldn't get involved without doing remedal work or re-wire

Bruthal

Its one of those ones, what do you do when they want 2 or 3 extra sockets, not do it unless its rewired? Or do the sockets, with their own new circuit and RCD etc?

New sockets with new circuit and their own RCD will work fine there without any neutralising. But what do you do, the rest of the installation could be bad. If someone just wants a socket plate changed, or a bulb holder, or a bulb, then what?

M cebee

ya

i meant the neutralizing side of it

Bruthal

Yes i know, i was just sort of asking my own question, saying that its one of those sort of,, what do i do here,,, ones.

grousedogtom

Yes i spoke with esb today and the supervisor seemed anxious to get it neutralized asap, so i just want my end right before this happens.

M cebee

i'd advise them to get the house re-wired or brought up to standard

before neutralizing

grousedogtom

OK i bonded all copper in hot press bathrooms and sinks etc , the wiring in the house is pretty old and in my opinion needs to be rewired but at the moment they are not in a position to do this. My dilemma is do i change to tn cs or stick with the present tt earthing system.

M cebee

not trying to be smart grousedogtom -as i've been there with customers and these old houses

but if nothing was bonded before

and you bond everything back to MET with no rcd on circuits

-it's now more dangerous than before

so prob try get them re-wire or forget about bonding

grousedogtom

Your not been smart i appreciate the advise as its not easy decide whats best in these ancient installations, if i fitted a 30ma rcd on sockets and 30ma rcd on ever thing else to ensure safety , i know this could be a nuisance if 1 trips effecting almost everything but at least safer.

M cebee

all i can tell you is


i'd play it safe ,and

- do complete re-wire or remedial work and certify house

or do nothing more except put 30ma on new sockets you've 'already' fitted