If i was asked to test some buildings to determin whether the Ze was to comply (dependent on the type of supply) and that i new there was no main bonding in the buildings (due to no water/gas services)would it be possible to take a Ze reading at a socket to see if the supply had a sufficent main earth.
i know the buildings installation should have already have been tested for this before they where energised!!!!!!! also im not looking for exact figure just need to determine if they comply.
[This message has been edited by james S (edited 08-26-2004).]
No! technically ze is what it says the external loop so you need to switch off and disconnect the outgoing earth and measure the loop of the supply cable. If there is a difficulty with isolation why not do Zs at socket and then do R1+R2 measurment on the same final circuit, subtract your R1+R2 from Zs and that will give you an approximate Ze. Hope it makes sense. Alan
Re: EARTHING#141501 08/27/0405:40 AM08/27/0405:40 AM
i see your point aland but if you where to take a Zs test and the resistance reading was lets say 0.5ohms for a tn-s supply system, this system would comply and when you take your R1+R2 measurment and subtract it away from your Zs the initial reading obtained can only get better.
James, your right, but in measuring ze you are trying to eatablish that there is a satisfactory external loop impedance. Dont forget that any zs measurment is likely to take the shortest possible path, exactly the same as an earth fault will, any fault will not travel the length of the cpc back to the dis board. If the building has a steel frame with metal conduit strapped to it it becomes a parallel path and will affect your readings, as will any water heaters or central heating connected. The only true way is to measure Ze with everything disconnected and then add R1+R2 to give you Zs. invariably the measured Zs will always be lower due to these parallel paths
Re: EARTHING#141503 08/28/0408:30 AM08/28/0408:30 AM
in my first post i stated that there is no main bonding, there for no parallel paths which means only the circuit cpc and external supply cpc are used for a return loop. i appreciate your points made aland thanks for your input.
Re: EARTHING#141504 08/30/0405:09 AM08/30/0405:09 AM
Measuring Zs and then allowing for the cable impedance will give you a true value for Ze only if you can be absolutely certain that there are no parallel paths to earth anywhere.
The lack of bonded water/gas services is a start, but there are many other places where a parallel path could exist. Alan mentioned conduit strapped to building metalwork.
Even if wiring is run as T&E or in PVC conduit you could still have a metal-clad socket or switch box in contact with structural steel somewhere. If you want to be really picky, even a metal box screwed into mortar will offer some resistance to earth (O.K., admittedly any such resistance is likely to be so high as to be insignificant compared to Zs).
Re: EARTHING#141505 08/31/0404:37 AM08/31/0404:37 AM
Good earthing is of up-most importance in any Electrical system. Ze = the impedance of the Earthing system, which should be of a very low value, to ensure that a large enough current flows under fault conditions. Zs, on the other hand, gives the Impedance of all of the Supply side of the circuit. Phase wires, the transformer winding(s), and the fusing through to the sub-circuit.