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Topic Title: RCD Time test and Fault Voltage
Topic Summary: maximum permissable values?
Created On: 05 December 2013 04:17 PM
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 05 December 2013 04:17 PM
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I've been learning to use the Fluke 1651B Multifunction Tester for a specific requirement on some MOD equipment and I'm having a small problem with the RCD Time test.

The test provides two results; Delta T which is perfectly understood, and Fault Voltage Uf which is defined in the user handbook as a voltage (N to PE) related to the rated residual current. No mention is made of the maximum permissable value but the values obtained by practical experiment have all been zero volts.

I have been able to find the references in the Regs and OSG for every test function with the exception of Uf. the only mention I can find is in "definitions" which then points to 442.1.2 which only repeats the definition.

Can anyone direct me to the part of the Regs or OSG which makes meaningful mention of Fault Voltage Uf in respect of RCD testing, such as max permissable or "must be less than.." etc.
 05 December 2013 04:31 PM
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I'd have a guess at 411.5.3 (ii) - although it's expressed is slightly different terms.
- Andy.
 05 December 2013 04:57 PM
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When measuring trip time for any type RCD, the
tester first does a pretest to determine if the
actual test will cause a fault voltage exceeding
the limit (25 or 50 V). To avoid having an
inaccurate trip time for S type (time delay) RCDs,
a 30 second delay is activated between the
pretest and the actual test.

This is from Flukes 165 user manual.

 05 December 2013 06:30 PM
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Having worked in electrical contracting for the MOD sector for many years, I know there may be numerous reasons for the meter to abort the test due to touch voltage.

What type of RCD device are you testing? The MOD quite often specify bespoke type units for live working areas that consist of an isolation transformer tapped at 120v-0v-120v with residual current detection on the secondary earth conductor only, via a CT (The earth conductor normally incorporates a 12K resistor inline to limit the fault current to earth to 0.01mA (for 5mA tripping device), thus telling your meter to abort)

Perhaps you could tell us some more information.

The Megger 1700 series is an excellent device for working in this sector as there are many functions that can be used. For example the meter has the capability to look at voltages between L-N, L-E & N-E at the touch of a button.


 05 December 2013 07:05 PM
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Sounds exactly the same as the isolating transformers i fit in schools,

Yes i do do 24/7 everyday of the FLAMIN year.
 05 December 2013 07:13 PM
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Yes similar units are often used in schools. FDB electrical specialise in these units. Testing of these RCD devices needs to to be carried out in a particular fashion as you would appreciate.

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