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Topic Title: Meter readings
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Created On: 20 November 2012 08:49 PM
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 20 November 2012 08:49 PM
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Avatar for ebee.
ebee

Posts: 5698
Joined: 02 December 2004

You know when you do an RCD test and set the meter to warn/cancel test at say 50 or 100 V on earth, how does it measure your earth?

In other words what does it reference it too to decide it is say 100V as you do not have another earth to the meter to compare it to at the time of the test.

In other words you say plug into a socket.

It checks L to N is 240
L to E is 240 or thereabouts
N to E is not much.

Then it tests (same on a Z test too) if the earth does get to 100V above true Earth is it because it compares to find it 140V below whatever L now is or does it compare it to the N perhaps.

How does it know?

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 20 November 2012 09:42 PM
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daveparry1

Posts: 6202
Joined: 04 July 2007

I would think the meter measures L-N and if that's say 240v but the figure between L-E is less than 190 v the earth level must be more than 50 volts? Not sure if i've explained that the way I intended!

Dave.
 21 November 2012 09:55 AM
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OMS

Posts: 19664
Joined: 23 March 2004

The meter does a quick Zs (or Ra) test at very low current (say 15mA) and applies a formula roughly akin to:

Measured voltage to earth/253V x Zs , where Zs is the very low current test it carried out without you knowing it.

The meter then determines from the above if the result is less than 50v, the test proceeds, if not it aborts the test

Regards

OMS

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Failure is always an option
 21 November 2012 01:49 PM
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ebee

Posts: 5698
Joined: 02 December 2004

Thanks OMS, I got that (I think) .

Reason I ask is someone was having a problem on a ("possibly home made" ) PME .

R2 just over 1 ohm Zs about 20 ohm and RCD test aborted given exceeding 100V error.

I surmised that one explanation might be a bad joint (high impedance) on the L side causing a voltage dropper effect and the meter could interpret that as if Voltage less under load than open circuit then the difference could be the E going high rather than the L going lower.

Turns out theres a dimmer in circuit so this could be the dropper.
I advised him to link out dimmer and retest.

My logic being, when say testing a socket, you got three connections.

A meter should be capable of measuring voltage difference between the three.

So open circuit it takes measurements then under a load (resistor integral to the meter perhaps) it retakes measurements.
Finds the difference(s).
It must assume one of those three is a datum (L perhaps) so a voltage open circuit of say 250 becoming 200 closed circuit could be interpreted by the meter as either the L reducing 50 volts or E increasing by 50 volts or both L & e moving closer together by 25 volts each

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

Edited: 21 November 2012 at 10:03 PM by ebee
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