Earth leakage meter.. Do I really need one?

[LastManOnline]

While waiting for a reasonably priced earth leakage clamp meter that will do both AC and DC leakage, I find myself asking the question which is the title of this post.And so tonight (been prompted by "Listers" Post) I thought "why not ask the forum? I have to date never not solved any faults to which I have been called to attend. Could I not continue without the earth leakage meter?. Or do I need one to make life easier?. If so., under what circumstances?

 

[Mikegh]

From my cursory glance through that linked article
What it was saying was the B type let's through 60ma of harmless DC current , whatever that means
Its not blinded by the DC I suppose is the important thing

Now how you go about checking and quantifying all these various leakages and testing is for the "higher ups" than me

Think I'll stick with the Megger and trial and error and multiple RCBOs for problem solving

 

[Mikegh]

From the article:

"Figure 23 illustrates that Schneider Electric 300mA type A RCDs can be used upstream a 30mA Type B RCD without risk of blinding: Type A protection of the device is guaranteed even in case of a 60mA smooth DC residual current. If the smooth DC residual current reaches the value of 60mA, the 30mA type B RCD will trip"

Seems to be an important point

 

[pc1966]

"Figure 23 illustrates that Schneider Electric 300mA type A RCDs can be used upstream a 30mA Type B RCD without risk of blinding: Type A protection of the device is guaranteed even in case of a 60mA smooth DC residual current. If the smooth DC residual current reaches the value of 60mA, the 30mA type B RCD will trip"

See also:

RCD "blinding" by DC | on ElectriciansForums

Find RCD "blinding" by DC Advice and Help. How-to RCD "blinding" by DC in the Electrical Wiring, Theories and Regulations advice boards on ElectriciansForums.net | Free Electrical Advice - Electricians Forums: Electricians' Talk Forums. RCD "blinding" by DC on...

www.electriciansforums.net

 

[Marvo]

20 years ago you could calculate the leakage on a circuit or an installation by running a 250v IR test and doing the math to work out the leakage current. Nowadays with all the standing functional leakage from switchmode power supplies and LED drivers the leakage current rarely bears any resemblence to the IR value measured as a dead test.

I'd say a leakage current clamp meter is an essential item and nowadays and leakage current must be measured as a live test using that device rather than trying to calculate it from an IR dead-test result.

I'm not sure a a clamp meter that measures AC and DC is essential unless you're working with a lot of type-B RCD's.

 

[newfutile]

You may want to look at note 2 here in the regs:
531 .3.2 Unwanted tripping
Residual current protective devices shall be selected and erected such as to limit the risk of unwanted tripping. The
following shall be considered:
(i) subdivision ofcircuits with individual associated RCDs. RCDs shall be selected and the circuits subdivided in such a way that any earth leakage current likely to occur during normal operation of the connected load will not cause unwanted tripping of the device. See also Section 314
(ii) in order to avoid unwanted tripping by protective conductor currents and/or earth leakage currents, the accumulation ofsuch currents downstream ofthe RCD shall be not more than 30 % ofthe rated residual operating current
NOTE1: ThiswillalsoallowabetterselectionofthetypeofRCDsaccordingtothenatureofthecircuitortheload.
NOTE 2: RCDs may operate at any value of residual current in excess of50 %of the rated residual current. (iii) use ofshort time-delayed RCDs, provided the applicable requirements ofChapter 41 are met

 

[Mikegh]

You may want to look at note 2 here in the regs:
531 .3.2 Unwanted tripping
Residual current protective devices shall be selected and erected such as to limit the risk of unwanted tripping. The
following shall be considered:
(i) subdivision ofcircuits with individual associated RCDs. RCDs shall be selected and the circuits subdivided in such a way that any earth leakage current likely to occur during normal operation of the connected load will not cause unwanted tripping of the device. See also Section 314
(ii) in order to avoid unwanted tripping by protective conductor currents and/or earth leakage currents, the accumulation ofsuch currents downstream ofthe RCD shall be not more than 30 % ofthe rated residual operating current
NOTE1: ThiswillalsoallowabetterselectionofthetypeofRCDsaccordingtothenatureofthecircuitortheload.
NOTE 2: RCDs may operate at any value of residual current in excess of50 %of the rated residual current. (iii) use ofshort time-delayed RCDs, provided the applicable requirements ofChapter 41 are met

A design figure of 30% makes sense I suppose

 

[Mikegh]

20 years ago you could calculate the leakage on a circuit or an installation by running a 250v IR test and doing the math to work out the leakage current. Nowadays with all the standing functional leakage from switchmode power supplies and LED drivers the leakage current rarely bears any resemblence to the IR value measured as a dead test.

I'd say a leakage current clamp meter is an essential item and nowadays and leakage current must be measured as a live test using that device rather than trying to calculate it from an IR dead-test result.

I'm not sure a a clamp meter that measures AC and DC is essential unless you're working with a lot of type-B RCD's.

I'm beginning to think you're right that AC measurement is probably adequate in most cases

The B types primary function seems to be to operate in the.same.way as standard rcd ie: trip somewhere between 15 and 30ma AC leakage

In addition it allows 60ma of DC leakage through without tripping and is not blinded by it

 

[Mikegh]

Can someone more knowledgeable than me explain whether true RMS measurement is needed for leakage current or why

Is it more accurate here or does it make any difference ?

 

[pc1966]

Can someone more knowledgeable than me explain whether true RMS measurement is needed for leakage current or why

Is it more accurate here or does it make any difference ?

Specifically for low currents as seen by an RCD I am not convinced it matters as I doubt that RCD actually respond to avegage(I^2 ) and suspect it is a simpler approximation.

However for a current clamp meter in general I would say true RMS is a very desirable aspect as it then directly relates to the heating effect of current for cable ratings and the overload protection aspect of MCB/fuses/etc.

Most folk won't want to keep two current clamps in their tool bag so having a meter that can read up to 100A for domestic / light industrial consumption use as well as down to mA for RCD leakage is better. Obviously if you need kA readings it is going to need a bigger clamp!

 

[pc1966]

A design figure of 30% makes sense I suppose

Usually you see a 3:1 ratio in cascaded RCDs along with a delay of roughly 0.2s more each time.

So 30mA "instant", then 100mA selective incomer for that board, and possibly a 300mA delayed one set to about 0.4s above that at a main DB, etc. Though often with the main ones they are adjustable MCCB style to allow for the possibility of greater leakage currents up to a couple of amps, etc.

I suspect it is not just about the typical 50-100% trip point of the upper once not being crossed by the 100% of the lower one, but also to allow for voltage disturbances on the supply (steps up and down on external faults or big loads being switched) as more dV/dt than the typical since wave causes more capacitive current, and also you might have checked the leakage at 220V and later the system is running at 245V, etc.