If I measure the voltage from the apparently live wire in the kitchen ceiling choc block to the the earth in the ceiling choc block will I trip the RCD? The CU has an RCD for the upstairs and downstairs rings and I tripped the downstairs one when I was fitting my new thermostat by allowing the neutral and earth to touch - even though the boiler is on its own circuit.
The other idea was to test the voltage to a known working neutral in the downstairs ring circuit.
I am beginning to think I have an open circuit neutral somewhere and if so I need to work back from the affected light until I find the break.
Voltmeters are designed to place as little load on the circuit under test as possible so that they don't significantly change the thing they are trying to measure.
Even my analogue meter has an input impedance of 4K Ohms/Volt when measuring AC voltage. So on a 250V range that will look like a 1 MOhm load, which will only pass 230 microamp - not enough to trip a RCD.
A digital meter will likely have a significantly higher input impedance (could be over 5 MOhm), so that would leak even less.
(this is why you can see phantom voltages when you measure a voltage disconnected wire to neutral - the meter does not put enough load on the pull the voltage down. The same effect can cause CFL and LED lamps to flash intermittently or even glow faintly when "off")
Yup that is common... Since there is typically a significant current passing through the live and neutral to the property, you will see a measurable voltage "drop" on the supply wires. Some of that drop will look like a rise in the neutral voltage. Connect that neutral to earth and you can get a significant current flow even though the actual voltage difference between neutral and earth is still very small.
(Notionally the earth and neutral should be at the same potential since they are joined together *somewhere*. However the distance from the join will have an impact).
With a TN-C-S earthing system (Earth Neutral bond in the main cutout just before your CU) you will see less potential between the neutral and earth since only the voltage rise in the neutral with respect to the earth, will only be that on your neutral meter tail resulting from the total load presented by your complete installation.
It is far more noticeable if you have a TN-S earthing system where the neutral to earth bond will be at the substation which could be could be some distance from the property. There you can see not only your locally caused voltage drop, but potentially some of that caused by other properties in the neighbourhood. (again in a perfect world where the load from all the properties is perfectly balanced across the three phases, the neutral current would actually be zero - but it unlikely that the balance is perfect!)
It can be instructive to work through some examples:
Say you have a TN-S supply and your supply impedance at the consumer unit is 0.25 ohms, and you had a total load on all the house circuits of
10A at the time you made the earth neutral short. 10A across a 0.25ohms will drop 2.5V.
That means (if we assume the L & N supply wires are of equal cross section), you would see half that drop in each leg of the supply. i.e. a 1.25V reduction in the live voltage and a 1.25V rise in the neutral voltage.
Now connect that 1.25V neutral to your earth, and a current will flow.
You could guestimate it - say your external earth impedance was 0.5 Ohm, and the neutral supply impedance in half the total supply impedance at
0.125 Ohms, that is a total of 0.625 ohms.
1.25V / 0.625 Ohms = 2A of earth leakage current!
Even with TN-C-S, you can see the same effect. If your meter tails are
25mm^2 copper, and are each 1m long. They would have a resistance of around 0.75 mOhms/m. Stick 10A load through those, and you will see a voltage rise on the neutral of 7.5mV. Short it to your PME earth with its resistance of say 0.35 ohms, and you will see 0.0075 / 0.35 = 21 mA of earth leakage - quite likely to trip a RCD - especially if it is seeing that in addition to some "real" earth leakage from other circuits as well.
Yup - it has all the hallmarks of a poor connection somewhere. It is also the most likely fault based on the symptoms you described (i.e. working then not working - several lights affected etc).
Can't find any reference to anything with that naming.
Is this a voltmeter, or a multimeter? Is it digital or analogue?
I have had another poke around and discovered where the CU feeds its cables into the eaves and it's reasonably accessible.
The cables then head under the floor of bed 1 picking up the garage, bathroom and study - where all the lights work.
It then goes across from study to kitchen and on to lounge - where lights do not work. It goes past the upstairs loo plumbing and I'm hoping it's fairly accessible in the eves that side of the house.
What I have to do is get a grip of my claustrophobia and the knowledge that the fibre glass is sure to kick off my eczema and have a further poke around, it's definitely looking more optimistic.
The meter is a digital multimeter and if the company was really "EST 1004" predated usable electricity by some margin!
OK, having the layout of the circuit certainly helps.
Can you not access the "ends" from the light fittings / ceiling roses etc?
While not as easy to see the topology from there, it involves far less loft romping!
So yup, it can give you phantom readings if you look at wires that are energised by nothing more than just a long run in close proximity to another live wire.
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