Darling....Why am I getting Electric shocks off the...COOKER

[ill_wind]

Seems ice cold rad man is off the forum now but he has a problem with the earthing in his property, the earth should be at the lowest potential so it should be impossible to get a shock from earthed appliances. Seems any faulty appliance in his house could make all the metalwork live looks like abit of a timebomb to me

 

[plugwash]

the earth should be at the lowest potential so it should be impossible to get a shock from earthed appliances.

lets get some basic facts straight

lowest potential is A MEANINGLESS TERM IN AN AC SYSTEM and anyway even in a DC system the direction of a potential difference is meaningless from a safety point of view

It is not possible to state the potential of any point in a system as a figure in its own right only the potential difference from a given reference

you get a shock

it should further be noted that AC voltages as well as having a magnitude can have a phase angle but once again this value is meaningless in isolation. It only has meaning to measure the phase difference between one potential difference and another potential differenct in the same (phase synchronised) system. This is what allows you to have 3 phases all 240V RMS from earth but 415V from each other

now lets look again at how we provide protection

EARTHED
EQIPOTENTIAL
BONDING
WITH
AUTOMATIC
DISCONNECTION
OF
SUPPLY

ok first equipotential bonding
we bond metal that either has some form of connection to earth or surrounds live electrics together to keep it at the same potential during a fault

second earthing
the main reason we earth the protective system is to prevent shocks between stuff connected to the protective system and the body of the earth

third automatic disconnection of supply
we obviously don't want fault current to flow for too long and equipotential bonding can never be perfect so we connect the protective system back to the neutral point of the generator or transformer either directly via cabling or indirectly via the earth. This causes current to flow when something shorts to earth tripping/blowing a protective device

from the point of view of electrical shock indoors Equipotential bonding (and note that in this sense the CPC (earth) cores of the cuircuit and appliance cables are part of the equipotential bonding system as well as the earthing system i suspect this is why calling them the earth cores is no longer considered correct) is FAR more important than earthing The system or provideing a return to the transformer star point

 

[ill_wind]

Blah blah blah, thats all boring mumbo jumbo as far as I can make out cannot see what 3 phase has to do with anything.

Q. Which way does the current flow?
A. To the point of least resistance.

Q. what is potential difference?
A. the difference in voltage between two points.

Q. so which way will the current flow?
A. from a point of higher potential to a point of lower potential.

Q. so in a safe system what would you want the earth to be?
A. at the lowest possible potential.

If the earth is not at the lowest potential how can you be sure enough current will flow in a fault condition to activate the circuit protection?

Seems to me that in ice cold rad mans house the people touching the appliances are at a lower potential than the appliance earth which is why they got electric shocks when the fault developed and also why no circuit protection was tripped.

If I was to make a judgement I would say he has no earth continuity on the supply side of his consumer unit.

As a side bar plugwash do you do any electrical work and if so how do you isolate the circuit you are working on?

 

[AdamW]

Reading and re-reading both your posts I can't see any real problems:

the earth should be at the lowest potential so it should be impossible to get a shock from earthed appliances.

It isn't entirely clear, but I am pretty sure Ill-wind is talking about equipotential bonding to earth here, i.e. the earthed appliance cases should have zero potential difference to any other earthed/bonded point. However, whilst you know what you mean (and I think I know what you are saying), it is quite possible someone will read this and start working on their electrics without really understanding the reasons behind it all.

Q. Which way does the current flow?
A. To the point of least resistance.

Although I do have to disagree with that statement, would kinda make Kirchov's First Law a bit redundant!

 

[plugwash]

Blah blah blah, thats all boring mumbo jumbo as far as I can make out cannot see what 3 phase has to do with anything.

Q. Which way does the current flow?
A. To the point of least resistance.

well current flows from points of higer potential to points of lower potential in amounts that vary with the potential differnce (voltage) and the resistance.

Q. what is potential difference?
A. the difference in voltage between two points.


Q. so which way will the current flow?
A. from a point of higher potential to a point of lower potential.

true however in 10ms time those points will have a potential difference in the opposite direction due to the way mains works

Q. so in a safe system what would you want the earth to be?
A. at the lowest possible potential.

there is no such thing as the lowest possible potential. the mains is varying constanly between 339V above earth potential and 339V below it ( peak=rms*sqrt(2) )

The reason we bond to earth is it is pretty difficult to completely isolate from it (especially for stuff outdoors) so it is the most sensible thing to choose for our equipotential point.

If the earth is not at the lowest potential how can you be sure enough current will flow in a fault condition to activate the circuit protection?

in a normal eebads system (which almost all systems are but there are some special cases mainly involving live working on equipment under repair where they are not) the earth and neutral are kept at roughly the same potential and so a short from a phase conductor to earth will cause current flow. in a TN system the resistance form the earth core back to the neutral system should be low enough to take out a fuse/mcb in a TT system the path has much higher resistance (because it has to run through the body of the earth) and hence other forms of protection must be used. Neutral-earth faults are not traditionally protected against and in most cases the potential difference between the two is small enough to keep the cuurent flow and hence the risk they pose down but there can be some pretty nasty issues with them in a few cases (especially where long high current TN-S systems are involved)

Seems to me that in ice cold rad mans house the people touching the appliances are at a lower potential than the appliance earth which is why they got electric shocks when the fault developed and also why no circuit protection was tripped.

If I was to make a judgement I would say he has no earth continuity on the supply side of his consumer unit.

there was definately poor continity in the earth system somewhere however unleess the person has good contact directly with the ground (which is pretty unlikely indoors) then they must have got the shock between two things which were at different potentials. Therefore i would think it is more likely caused by the combination of BOTH missing main earth and main equipotential bonds OR a missing earth in the cooker cuircuit . The latter seems the more likely to me although both are possible

As a side bar plugwash do you do any electrical work and if so how do you isolate the circuit you are working on?

only diy (iim currently studying electronic systems engineering at university btw) in most cases i normally just turn off the mcb because turning off everything would be too disruptive to the rest of the family. I know its only single pole isolation but with the way recs do thier networks now the risks from a broken neutral in thier network are pretty damn small it is annoying to take out the rcd when you touch both neutral and earth cores with your tools at once by mistake tough.

 

[ill_wind]

If they had good contact with the ground they may well have received a fatal shock!
While you are working on your partially isolated circuit there is mains voltage on the neutral as all the neutrals are connected together at the consumer unit, the only reason you do not get a shock is because it has an easier route returning via the incoming supply than through your body which is exactly the same way that the earth should work.

 

[AdamW]

Eh? Are you sure that's right? I can see what you mean (neutral is a return path thus current flows along it), but to say that neutral is at mains voltage is incorrect.

Neutral should be kept as close to earth as is possible on a domestic supply, the reason you (hopefully) don't get a shock off neutral is that there is very little in the way of p.d. between neutral and yourself (earth).

If your incoming neutral were disconnected then the neutral conductors could go to mains voltage, but otherwise neutral is at effectively 0V. Hence the name "neutral"! If it were at mains voltage then you would get no p.d., therefore no transfer of energy.