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Discuss Leakage current meter - recommendations? in the Electrical Tools and Products area at ElectriciansForums.net

l4urence

Regular EF Member
I'm looking at buying something for measuring leakage current - The Megger DCM305e (I think). Not specifically for nuisance tripping but more for DC leakage current - type A vs AC RCD (type AC flooding with DC and not tripping). Not a lot of info on the DC side - except DC units for PV, I understand the AC leakage sits on top on the DC value - so say 2mA AC leakage, and 2mA DC leakage - would show 4mA? if the meter cannot 'see' the DC then it's not what I need. Any thoughts, comments or suggestions are welcome.
 

TJ Anderson

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Mines a cheapo from ebay. AC only. Had it for years. Was about 70 quid back then I think. Is accurate though. A mate at work has an earlier megger offering. I have compared readings before and virtually identical.
 

l4urence

Regular EF Member
So, I already have a cheap clamp meter. The exam question here is how can I prove that a type AC RCD will be rendered useless by the DC leakage current - I could be reading AC leakage which is fine. How can I possibly recommend a A type if I cannot measure the 6mA DC threshold leakage current.
 

PEG

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Hi,i was having one of those DCM305e jobbies,thrust at me,during a recent tool fair....i don't recall the DC part being mentioned.
 

l4urence

Regular EF Member
Hi,i was having one of those DCM305e jobbies,thrust at me,during a recent tool fair....i don't recall the DC part being mentioned.
I’m the in the same boat, I thought it did but haven’t read anything more - market appears to be for overtripping AC leakage but I’m interested in seeing whether the loads connected have DC leakage that could prevent the AC type operating. It may well pass the RCD testing but when the family is home and watching TV, computer, games and charging through USB sockets - all electronic devices - are they now in danger?
 

PEG

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Have a look at what is in Megacons range...get your wallet dubbined;)
 

spinlondon

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Can you not just test the RCD?
If it works there’s no dc current saturating the coil.
If it doesn’t work, change it.
 

l4urence

Regular EF Member
Can you not just test the RCD?
If it works there’s no dc current saturating the coil.
If it doesn’t work, change it.
Thanks @spinlondon Exactly - that’s where we are but the dc leakage will come from usage, so if during the test equipment is off, say people are out it’ll pass. Was thinking of a test extension lead with the earth looped out for the clamp and monitor DC leakage on equipment. I’m trying to verify that the leakage is high enough to justify and recommend an A type over AC based on the equipment being used.
 

AJshep

Electrician's Arms
I saw this also when looking through the BBB (Regulation 531.3.2/3). I'm also considering buying a earth leakage clamp meter.

The way I read it we have two issue's..
If we are considering changing a consumer unit for a dual RCD type we should measure the earth leakage beforehand to check the accumulation of earth leakage and avoid nuisance tripping. The only way is using a earth leakage clamp meter.
Then secondly is the point you mention above about DC leakage currents causing AC type RCDs not to operate.

IMO In most situations it's pretty rare that on a single circuit there is enough DC earth leakage to cause a RCBO to operate, even if its a AC type. However, a bunch of circuits covered by a RCD in a split load CU could cause problems.

I generally just play it safe and install A type RCBO CUs anyway because without measuring there is no way of knowing.
 

l4urence

Regular EF Member
I saw this also when looking through the BBB (Regulation 531.3.2/3). I'm also considering buying a earth leakage clamp meter.

The way I read it we have two issue's..
If we are considering changing a consumer unit for a dual RCD type we should measure the earth leakage beforehand to check the accumulation of earth leakage and avoid nuisance tripping. The only way is using a earth leakage clamp meter.
Then secondly is the point you mention above about DC leakage currents causing AC type RCDs not to operate.

IMO In most situations it's pretty rare that on a single circuit there is enough DC earth leakage to cause a RCBO to operate, even if its a AC type. However, a bunch of circuits covered by a RCD in a split load CU could cause problems.

I generally just play it safe and install A type RCBO CUs anyway because without measuring there is no way of knowing.
Good answer - you understand exactly. There is no way of knowing and your 'playing safe' is my view too. It would be nice to be able to know and prove it. I'm not concerned that the DC would trip the AC type - I'm interested to see whether the DC leakage would flood the AC coil and prevent it from operating against an AC fault.
 

l4urence

Regular EF Member
This is my point of interest:

http://www.doepke.co.uk/download/Techpub-17

"Type A RCDs are not suitable for detecting smooth DC residual currents since they result in pre-magnetization of the summation current transformer and therefore significantly impair functionality. Based on the relevant product standards, the maximum load which may be applied to Type A RCDs in the event of a fault is 6 mA DC, regardless of their rated residual current"

so - the 6mA DC - measured... how?
 

AJshep

Electrician's Arms
I'm not concerned that the DC would trip the AC type - I'm interested to see whether the DC leakage would flood the AC coil and prevent it from operating against an AC fault.
The Hager rep I spoke to a while back seems to think so. He told me that over the last few years RCDs have been returned to them as faulty, when tested back at Hager they found that they are not. Hager put it down to DC leakage stopping the AC RCDs working, this apparently us why their split load boards now ship with A type RCDs.
I've never seen it personally, and it was only a rep I spoke to.
In theory it could happen I suppose.
 

l4urence

Regular EF Member
@spinlondon Agree absolutely. I just wanted a way to prove it - with a measurement. With more and more electronics being used on domestic circuits, I believe this will become important, since an AC or indeed A type RCD may not be protecting them but passing the tests. Don't get me wrong here, not wishing to scare tactic selling RCD's, just interested when regs / tech points at a solution (like AFDD's...) with no way of proving or testing!
 

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