Earth Impendence test

[joshjames]

What is an earth impendence test and how do you do it?

 

[ricicle]

It is a measurement of the Earth Fault Loop Impedance, which is the path that fault current will take when a short occurs between Line and Earth. It has to be sufficiently low enough for protective devices to operate correctly.
It is measured at various points around a circuit including at the intake position using a earth fault loop impedance tester. Protective devices have to be rated to be able to break higher values of fault current.

 

[xr4x4]

What would cause a high reading?

 

[RF Lighting]

Copper wire has a certain resistance per meter depending on the cross sectional area of the wire.

The smaller the wire, the higher the resistance. Resistance limits current flow, and if the current flow is not high enough during a fault, the fault may not be automatically cleared by the fuse / circuit breaker / RCD quick enough.

High readings can also be caused by high resistance joint(s). Theres can be caused by loose screws in accesories / junctions, corrosion, snapped wires, rusted steel work to name a few. This could be on the consumers installation, or on the supply network, which is why EFLI testing is so improtant.

 

[Taylortwocities]

What is an earth impendence test and how do you do it?

Here ya go

http://www.tlc-direct.co.uk/Book/8.6.2.htm

 

[bernardgreen]

Isn't there a minimum test current that should be used when measuring earth fault impedance ? And isn't this minimum current many times the current that a general purpose multi-meter uses to measure resistance ?

 

[Adam_151]

Isn't there a minimum test current that should be used when measuring earth fault impedance ? And isn't this minimum current many times the current that a general purpose multi-meter uses to measure resistance ?

I'm not so sure there is, the older testers used 24A as they just had a ten ohm resister, but more modern testers use a lower test current, the data sheet for mine informs me that it uses 15mA for non rcd tripping test (but this range isn't amazing for accuracy), and 4A for normal high current testing (pretty accurate and repeatable)

AFAIK I know most instruments take a few readings and average them out to loose as much of the error as possible (even on the normal range*, as well as the non trip range [which does a pretty large number as the low current sued is prone to error])


*Its noticeable when you've got a rather long lighting circuit thats probably a bit out of spec for volt drop, you can see it flicker a good few times as the reading is taken