Electricians' question

[The Builder]

Is it possible to measure earth current leakage on a Megger, or is a clamp meter the only way?

[Hymie]

The problem with using a clamp meter is their accuracy at low currents. The most accurate method is to break the earth path and insert an ammeter in line. This technique could result in a hazardous situation with the meter impedance (albeit quite low) in the earth path.

Most PAT testers will perform this measurement on individual appliances – but I guess your question relates to wiring installations and the earth leakage due to the electrical equipment within the building.

[BRIGHT SPARK]

Hi TB...you need to get an earth leakage clamp meter..they will measure quite accurately in Ma......you could do as Hymie suggests but put your wellies on......or do as the regs say...get the lad to do it ;-D

[The Builder]

Ha......... yes, I've heard the stories of the exploding ammeters and frazzled apprentices :o)

Thanks fellas. Just wanted confirmation.

[Hymie]

I stand corrected by Bright Spark.

I regularly use clamp meters in my work, mostly in the 1A-50A range and was not aware that they are now available with mA ranges – this one looks just the job for a sparky wanting to investigate earth leakage circuit breakers breaking.

http://www.ebay.co.uk...0&hash=item33675d676f

But I don’t believe it has an accuracy of 2% on a 2mA range (as claimed).

[The Builder]

Thanks again Hymie. Much appreciated.

[Hymie]

Having said the above product would be suitable to investigate nuisance tripping of ELCB’s – one additional feature (not on this product) would be the ability to display (hold) the peak current. This would allow confirmation of a suspected current spike, causing the trip to operate.

Something to bear in mind is that ELCB’s operate by detecting an imbalance in the current between the supply and return conductors – quite a neat trick when you consider that they are detecting a difference of 30mA in a total load of 30A (they do not directly measure the earth conductor current). Some are prone to operate due to the intermittent operation of inductive loads (such as a drill), possibly due to the resulting electrical noise.