Voltage Between Microwave And Sink?

[TheDutyPaid]

Today in the kitchen I happen to touch the microwave and sink at the same time and got a shock.

Pulled out the multimeter and measured 103V AC between the two, also between the kettle it was the same, testing between the washing machine and sink I got 34V AC.

I live in a block of flats so wonder if someone else have a earth fault making the pipes live or the earth in the kitchen is not connected?

Can not believe my microwave, kettle and dishwasher have are all shorting to earth.

[wasabi]

I'd guess earth fault somewhere else in the property. Especially if the positive end is connected to the sink you get a positive voltage reading? If rented I'd call the landlord urgently either way.

[TheAnimus]

Do you have an RCD or Earth Leakage trip? Are the electrics in good order?

You are measuring a potential difference (as that is what voltage is). The fact many products have this when looking at the sink suggest that the sink is somehow connected to some power. Have you tested the sink to the earth pin in a socket?

If you can switch off all the ring mains one by one, if when you've switched the master off (or pulled the fuses) and you still have any PD (voltage) then it is much more serious.

It is quite possible you've one device that is leaking to earth. Now depending on the device in question it could be mostly harmless and the fact the properties piping is insulated from ground to blame. If you can narrow it down to one ring main, switch of all the sockets there, enable them half at a time until you find the one you are wanting (this is faster than doing them one at a time, see Binary Search for why!).

It could also be really dangerous so if you feel out of your depth, get a qualified sparkie in.

[wasabi]

The fact many products have this when looking at the sink suggest that the sink is somehow connected to some power. Have you tested the sink to the earth pin in a socket?

Wiring regs require sinks to be earth bonded as far as I know - not strictly so much for kitchen sinks but I believe most sparkies do it anyway.

[peterb]

In a Protective Multiple Earth bonded system most installations in the last 20 years, the earth connection is derived at the consumer unit from the neutral pole of the incoming supply. There will be an RCCB (Residual Current Circuit Breaker) which will disconnect the supply in the event of an imbalance between the line and neutral current flow - typically 30mA (although outdoor individual RCBs are typically 10mA). Earth and neutral are also bonded at various points back to the substation.

However on older installations it wasn't unusual to find a small voltage between earth and neutral because of the voltage drop across the neutral conductor.
You are measuring a relatively high voltage, although it appears to be a high impedance source - a current of 4mA to 10mA is sufficient to kill, and a voltage of 130 is sufficient to drive a current of 4mA through the human body (dry skin resistance is roughly 250KOhms to 1MOhm unless the voltage is from a high impedance source.

So I suspect you are detecting capacitive leakage across the conductors in either a power supply somewhere (switched mode "wall warts" are notorious for this as the low voltage appliance they supply don't usually have an earth bond) or the cable conductors.

Either way, it does point to a defect in the overall earth bonding of the flat. All pipework and metal objects should be bonded to earth - the reason being that in the (very unusual) occurrence that a neutral break occur AND a line to earth fault occur, all the objects in the premises will be at an equip-potential minimising the risk of shock, and also ensuring the RCCB will trip.

It needs to be investigated - either by your landlord if it is rented, or by an electrician if not. One relatively common cause for the integrity of earth (or more correctly - equip-potential) integrity to be compromised is the installation of plastic pipework, which is why thinks like sinks should have a separate earth bonding point.

I'd guess earth fault somewhere else in the property. Especially if the positive end is connected to the sink you get a positive voltage reading? If rented I'd call the landlord urgently either way.

The mains supply is alternating current, so the terms "positive and negative" have no meaning in this context. The terms would be line and neutral, but multimeter will not indicate which is the live pole and which is the neutral pole - only the fact that there is a potential difference. A multimeter would only give that indication if one side is connected to a known and verified earth point, and so anything measured would be relative to that known and verified point.

[TheAnimus]

Wiring regs require sinks to be earth bonded as far as I know - not strictly so much for kitchen sinks but I believe most sparkies do it anyway.

I found vulcanised rubber in one place I was renting once.

I generally don't approach a fault in electrics as if it has been wired to code

What I was guessing is somehow the sink isn't earth bonded, someone might have assumed something would be, but someone used PVC or changed to PVC piping. Meanwhile, something is leaking to earth, having seen some have-a-go electricians works, this wouldn't surprise me.

[wasabi]

In a Protective Multiple Earth bonded system most installations in the last 20 years, the earth connection is derived at the consumer unit from the neutral pole of the incoming supply. There will be an RCCB (Residual Current Circuit Breaker) which will disconnect the supply in the event of an imbalance between the line and neutral current flow - typically 30mA (although outdoor individual RCBs are typically 10mA). Earth and neutral are also bonded at various points back to the substation.
.

True. I was thinking the flat could be on one phase of a 3 phase system and the variations being the (derived) neutral point shifting in response to load elsewhere in the building. However still surely indicates an earth leak somewhere?

[peterb]

True. I was thinking the flat could be on one phase of a 3 phase system and the variations being the (derived) neutral point shifting in response to load elsewhere in the building. However still surely indicates an earth leak somewhere?

Yes - the incoming feeder to the flats will almost certainly be three phase, star connected, with the neutral being the star point. If the load in each phase is balanced, there will be no current flow in the neutral conductor. In a domestic situation, the loads will not be balanced, and there will be a current flow in the neutral conductor, and therefore neutral would not be at zero potential - were it not tied to ground potential by multiple earth bonds.

[TheDutyPaid]

Thanks for the help, I am still alive so far. I own the flat so no landlord to moan to, I have been here for about 15 years and the building dates form the late 1960's

There is no bonding under the sink or RCA's earthing is a nightmare the light sockets do not even have a earth, so no fancy metal plates for me.

Reading up on earthing does make me wonder what to do. There is a fuse box inside the flat but the main meter and power in, is in a meter cupboard with 6 other flats and is about 10 meters away from the fuse box.

I tested the sink to a earth point on the wall socket and it is still 100'ish V AC

To me the trouble is paying for a sparky to come in and figure it out. I am trained in electronics but only did a GNVQ in mains house wiring 17 years ago.

Will do some testing with the fuse box turned off to see if it is just within the flat or something else external.

[peterb]

It wasn't unusual for there to be no earth on lighting circuits, after all, if there are no exposed metal components, there is nothing to earth. However, under equipotential bonding, the metal boxes that contain switches are bonded so the heads of the metal screws (that are exposed0 are bonded - metal faced switches or metal light fittings are another matter) but modern installations will have twin and earth cabling on the lighting ring mains.

As for your problem, that tends to look at earth leakage - the current that is available is very low - the question is where is the 'true' earth. You could measure the PD between earth and neutral - that should be zero or very close to zero.

You might also start by unplugging every portable device in the flat (and isolating anything electrical that is fixed - immersion heaters and cookers should have dual pole isolation switches. If that doesn't remove the problem, isolate the ring main(s) at the fusebox and test again. If the problem persists, switch off the main incoming switch and test again. That may enable you to determine a faulty appliance, or a circuit where there may be a fault - at least within the flat. If it is outside the flat, it is either a problem for the block management organisation (if there is one) and/or your electricity distribution company)

The sink may have relied on it's earth bonding through the taps and pipework - if there isn't an earth bonding point on the sink, then you could check the continuity from the sink to the tap pipework, and if that is good, look at bonding the pipework to earth. I think the recommended bonding wire should have a cross sectional area of at least 2.5mm as in theory it could carry a high fault current.

Having done that, you should ensure the earth bonding integrity of every fixed metal item in the flat - bathroom taps, copper water cylinder and so on. Bathroom and kitchen are the two most important areas because of the use of water - wet skin is obviously a better conductor than dry skin, and the risk of a fatal shock is that much higher.

But you really should be getting a qualified electrician do do this if you are uncertain. While I doubt there is any immediate danger, there could be a hazard in a future fault scenario, and it is your life that could be at risk.