Is there a fault and what is it??????

[electricsrule]

Hello lads,

Right here goes I get a call of a plumber who's on a job doing a boiler chop..... He calls saying I've just tested the boiler spur and I'm getting strange readings.
So being the helpful sparky I am I plod along over to his job I have a quick look at this spur must be 15 years old and the connections on the back has the feed and load neutral in the wrong terminals so I think change the spur reconnect the correct way and solved happy days.........WRONG
The readings I'm getting are

feed line to neutral 230v
feed line to earth 230v
feed neutral to earth 0v

Load line to neutral 0v
load line to earth 0v
load neutral to earth 0v

So all seems well but if I go

Feed live to load neutral I get 185volts
feed live to load live I get 185volts

This is with the load dropped out of the spur

Why am I getting this voltage any ideas as heating systems are not my thing really I don't get any voltage at the wiring centre and this is a PME supply?

Ive spoke to another sparks and he said that its normal as its a PME you can have this as its a shared neutral earth........ He said its fine just leave it the system was working fine before??? I undestand its shared but there shouldnt be a voltage on a dead load from a live circuit apart from live to earth
This not normal so I don't believe what he's on about.


Any help would be appreciated maybe it's just my lack of understanding and experience.

 

[Brightspark9]

Could possibly be a floating neutral within the heating system.

 

[davesparks]

Could possibly be a floating neutral within the heating system.

Can you expand on this and explain what you mean and how it would create those voltage readings?

 

[Taylortwocities]

If the load has been removed, it sounds like the FCU needs replacing.
Probably switch contacts burnt/dirty.

 

[snowhead]

As above.

No load connected?
Spur switched on?
Neon?

Spur is U.S,
Failed because it's not been switched off for years or full of plaster and brick dust, or water

 

[Hellmooth]

If the load has been removed, it sounds like the FCU needs replacing.
Probably switch contacts burnt/dirty.

I'd say the same, if there isn't any load in the terminals then it isn't connected to anything, so must be a dodgy spur getting voltage from the feed side.....?

 

[marconi]

Hello lads,

Right here goes I get a call of a plumber who's on a job doing a boiler chop..... He calls saying I've just tested the boiler spur and I'm getting strange readings.
So being the helpful sparky I am I plod along over to his job I have a quick look at this spur must be 15 years old and the connections on the back has the feed and load neutral in the wrong terminals so I think change the spur reconnect the correct way and solved happy days.........WRONG
The readings I'm getting are

feed line to neutral 230v
feed line to earth 230v
feed neutral to earth 0v

Load line to neutral 0v
load line to earth 0v
load neutral to earth 0v

So all seems well but if I go

Feed live to load neutral I get 185volts
feed live to load live I get 185volts

This is with the load dropped out of the spur

Why am I getting this voltage any ideas as heating systems are not my thing really I don't get any voltage at the wiring centre and this is a PME supply?

Ive spoke to another sparks and he said that its normal as its a PME you can have this as its a shared neutral earth........ He said its fine just leave it the system was working fine before??? I undestand its shared but there shouldnt be a voltage on a dead load from a live circuit apart from live to earth
This not normal so I don't believe what he's on about.


Any help would be appreciated maybe it's just my lack of understanding and experience.

By Marconi In situations like this one needs to remember a few items of theory and relate them to the wiring you are studying.

This is what I discern from what you have written and what else I would do:

a. You found a wiring fault and corrected it - good. You replaced an old FCU with a new one (but did not completely describe it in your post- does it have a neon?) Next step is to confirm the connections at the other end of the spur and then from this junction all the way back to the distribution board. You have now proved continuity of LNE conductors along this path. Confirm correct connections and cable protection for cable size and installation method. What fuse in FCU - 3 Amp?

b. Take a good look at the cut-out, earthing conductor and main earth terminal to establish the type of supply and confirm the earthing system; TT, TNCS or TNC. (These are facts you want to know for any installation.) Confirm equi-potential bonding by observation and continuity checks. For example, I would measure the resistance between the CPC at the end of the spur and the copper work of the boiler. I also have a battery-buzzer- croc clip gadget to pass more current than the typical ohmmeter.

c. Check polarity at FCU for feed supply - across LN, LE and NE. You did this and found correct. You also found this to be correct with the load connected and the FCU switched on.

d. Now your 'strange readings when the load (I assume L, N and E) are all disconnected. You read a voltage of 185 Volts Lfe-Llo and Lfe-Nlo: what you are measuring is the potential difference between an energised live conductor, Lfe and an isolated, insulated terminal (Llo or Nlo) within the FCU. Well, you know the potential difference of Lfe with respect to the supply N and also E - each is 230 Volts because N and E are bonded electrically (at the installation if TNC or at the supply transformer if TNCS or electrically connected through the mass of earth between the supply transformer star point earth plane and the local earth rod.

e. Now ask yourself what is the potential difference between the FCU Llo and the main earth terminal and Nlo and main earth terminal. Is it zero? It can only be zero of there is a conductive path between these terminals and the MET and no current flow through it. There is not such a connection because there are no connections to these terminals . So the potential of these terminals with respect to the MET depends on these insulation resistance and capacitance between them and the live energised conductors Lfe. The insulation resistance may be high and the capacitance may be low(and thus reactance high) but they are not zero- - they are not completely insulated resistively and reactively - isolated There is also the high resistive and low capacitive (very high reactance) linkage between Llo and Nlo terminals and all the metallic and non-metallic objects near them. Thus, there is a high impedance path between them all the way to the MET. Again,the potential difference can only be zero if no current is flowing.These terminals are not completely isolated to the flow of electrons - electricity

f. What you should have conjured up in your mind is an energised supply connected across a high impedance between Lfe and each of the Load terminals and an even higher impedance between these terminals to the MET - a classic two series impedance potential divider circuit with the terminals

g. I leave it to you to explain why you read 185 volts on your voltmeter. Draw a diagram. If you know the impedance of your voltmeter you could even work out what the impedances of the potential divider circuit. Why are they the same voltage?

Enjoy your work and work safely. And keep learning. I like the addage that an educated person is one who realises how much he does not know (or has forgotten) and does something about it.