Why, on an electrical supply to a house with PME does the earth loop impedence have to be 0.35 ohms or less?

Answer 1

The earth loop must be low impedance in order to provide a low-resistance discharge path for current which flows under fault conditions (i.e. a short circuit). This low-resistance is necessary so that the fault current is high enough value to trip the protective device (fuse or circuit breaker) quickly.

The fault current is calculated by:
I = Uo (Nominal Supply Voltage) / Zs (Earth Fault Loop Impedance)

Example: The Zs of an installation with a 230 V supply is 0.8 ohms Therefore the fault current in a phase to earth fault connection is: = 287.5 A

So, as the earth loop impedance increases, the amount of current available to trip a circuit breaker or "pop" a fuse decreases. This increases the time to trigger the protective device.

There's a very good tutorial with practice exam questions at the link in "Sources".

Answer 2

pme is old name and not used any more
tncs
which means 2 wires come into property and one of the wires is used as the neutral and earth (NE conductor).

tns systems can have an impedance of 0.8 (i think).

the reason tncs has to be less is because the of the NE conductor

if the system takes a large amount of current then the potential of the NE conductor will raise (ie if 1 A then v= 0.35V/2 (since the impedance is that of the neutral and the live conductors)) etc

if it increases too much this could produce a danger.

to reduce the resistance the NE conductor is earthed in many locations along its length. but also a higher resistance could indicate that any of the earths have failed.

hope this makes sence

Answer 3

The earth loop impedance has to be low enough so that under fault conditions there is enough fault current to trip the circuit breaker within the correct time which is 4 seconds for fixed appliance circuits IE Lights, Cooker and 0.4 seconds for portable appliance circuits IE Socket outlets. (An electric shower is the exception this is fixed but should disconnect within 0.4seconds).
The maximum values for different circuit breakers and fuses is given in BS7671 The wiring regulations where the curves can be found as well.
Th 0.35ohms suggests that you are looking at the figure for socket outlets.

Answer 4

The overriding requirement is that sufficient fault current must flow in the event of an earth fault to ensure that the protective device (fuse/CB) cuts off the supply before dangerous shock can occur. For a normal 240 V socket outlet circuit its 0.4s. E.g. for a 20A BS3036 fuse to trip in <0.4s (~130A to blow fuse in 0.4s) the following earth-fault loop impedance is required : Z=240/130 so 1.8 Ohms.....

Answer 5

The ground at the switch box is to protect from lightning. I have Sean 2 different system that failed to meet your specifications one was in the desert ,even with a 8 ft. ground rod after a long dry spell there was 10 meg ohms between the box ground and the transformer ground not good. The other was high in the Ecuadorian mountains where it had rained so much that the distilled water between the box ground and the transformer was 50 meg. ohms. In both cases we had to run a cable tying the box ground and the transformer ground together.

Answer 6

No but it makes me have a good cry every once in a while. Lately i've been getting really stressed out about our family finances. I feel like a loser because we can't seem to keep our heads above water. It seems that our bills are rising by the day. Between paying for higher gas and food prices, we're really stretched. Financially and spiritually speaking, I wanna scream every time I get a bill.