mA clamp meters

[RF Lighting]

Does anyone here have any experience with these?

I was called to an RCD fault yesterday on a split load set up.

It was a 30mA RCD feeding all the sockets, shower, imm heater and central heating.

I ramp tested the RCD in isolation and it tripped at 22.5mA, then re-tested with the installation connected and it tripped at 1-2mA.

I then tested each circuit individually. A couple of circuits tripped the RCD at for example 19mA = 3.5mA for that circuit and the total leakages added together was 13mA. This shouldn't cause any problems for the RCD.

My first thought was a N-E fault with the additional load of the other circuits driving more current through the fault, so I broke down the installation and IR tested L&N>E of each circuit. The lowest two readings were 6.5MΩ and 68MΩ. The rest of the circuits were all ≥100MΩ so whilst not ideal, nothing that should really cause any concern.

I then tried to connect my multimeter between the earthing bar and the main earth with it set to AC mA range, but I have reason to doubt the readings were accurate as they did not tally with what my calibrated MFT was saying on the ramp tests.

I don't know if this was just due to a crappy multimeter or an incorrect method of testing.

Now to my question. I've seen my wholesalers sell a mA clamp meter. How do these work, and would they be any use for tracking this sort of fault?

Do they give a highly accurate reading when clamped to the line meter tail or do they just give a mA reading when clamped to the main earth conductor?

My immediate thought with this is that if it only reads the mA current in the main earthing conductor that it would be of no use for tracking leakage faults if the earth leakage is to true earth such damp penetrating an external or underground joint.

Sorry for the long post, but I though it would be more helpful to give you the thinking behind looking to purchase one of these testers.

 

[Adam_151]

I have one, towards the lower end of the market, and its useful to a point.

The idea is that you clamp it round all live conductors of a circuit (or a submain) and it looks at what is going astray in a similar way to what an RCD does. I've found it gives readings that are a bit different to doing it manually as you did with the ramp test on the RCD tester... but who is to say which figure was the right one :shrug:

It can be useful on EICRs as either a quick and dirty alternative to a whole board insulation test if you can't turn off, although its not conclusive (If you see 20ma going astray in a large office with a lot of IT, its probably not a problem. If 2A is going missing on a factory lighting board, then there is probably something wrong!). Or as a pre test to a whole board IR when you have a limited window to do the actual test. I can remember one instance where I went in to do the global IR knowing "The board will quite likely be down, but the problem is likely to be lighting circuit 5L3 (or whatever) so take that out and try it again" Sure enough it was spot on.

It is important to bear in mind though. That on a three phase supply, the earth leakage currents will cancel in the same way as neutral currents, so that's another thing machining the results less accurate

 

[EFLImpudence]

Just to say the current on the Earthing Conductor does not tell you anything as it can be coming from somewhere else or the leakage is finding another path.

If you clamp the tails, it shows, as said, the leakage of the whole installation including the non-protected side, so you could do the reverse of your testing by switching on appliances and/or disconnecting and reconnecting circuits.

You really need to clamp the L & N of each circuit but this is seldom possible with the usual rat's nest.

Whilst the mA meter is a good tool in a perfect world it is, I have found usually not practical because of the layout of the conductors.