tt system.

[Lenny07]

Hi all testing a tt domestic installation this morning, my main ze was 18.2 ohms and pfc was 13.3 amps. My concern is when i test zs on each circuit my megger is set on no trip, [dual rcd] the display shows trp but rcd does not drop out. When i set it off no trip the rcd trips out which i would expect. Im only using two leads live and earth, should i be using three leads or is there something else im missing? cheers everyone.

 

[Apache]

Thought you needed three leads for no trip? Ignore me tho

 

[Lenny07]

i was taught 3 lead on tncs and 2 on tt and tns! my mentor may be wrong though and this is what annoys me because you only learn what u have been taught!! He may have shown me everything wrong? :|

 

[sparkytim]

3 leads for low current test

Reason is it tests L-N loop and compares with L-E loop to come up with a reading.

 

[Lenny07]

Are we measuring the external fault path for each circuit back to the transformer then and why does it have to compare between n and e? If this is a silly question tel me to beat it but the info would be much appreciated. :Salute

 

[sprocketflup]

I used to use a Metrel multitester that only used 2 leads, seemed to work fine?

---------- AUTO MERGE Post added at 13:20 ---------- Previous post was at 13:18 ----------

Actually scratch that, just remembered the earth and neutral leads plugged into each other giving the impression of 2 leads

---------- AUTO MERGE Post added at 13:20 ---------- Previous post was at 13:20 ----------

Actually scratch that, just remembered the earth and neutral leads plugged into each other giving the impression of 2 leads

 

[kme]

If you have a copy of the current "connections" magazine, there is a 4 page spread in the back on the operation of various types of RCD; and the process by which loop impedance is measured in different ways - may be worth a look, as they do explain the various (high current; d.c. saturation, low current) modes of testing; and the effects on different types of RCD.

KME

 

[m4tty]

Is this 'connections' magazine an NICEIC thing as I cant find it on google mate. Any links if its available online? cheers mate

 

[sparkytim]

If you have a copy of the current "connections" magazine, there is a 4 page spread in the back on the operation of various types of RCD; and the process by which loop impedance is measured in different ways - may be worth a look, as they do explain the various (high current; d.c. saturation, low current) modes of testing; and the effects on different types of RCD.KME

+1

Giving away our sources ROTFWL

Excellent material.

---------- AUTO MERGE Post added at 16:05 ---------- Previous post was at 16:01 ----------

Are we measuring the external fault path for each circuit back to the transformer then and why does it have to compare between n and e? If this is a silly question tel me to beat it but the info would be much appreciated. :Salute

All to do with how the processor works inside it i believe

Strangely the NEW Megger 1700 is being advertised as Only needing 2 wire Non trip loop test.

---------- AUTO MERGE Post added at 16:07 ---------- Previous post was at 16:05 ----------

Is this 'connections' magazine an NICEIC thing as I cant find it on google mate. Any links if its available online? cheers mate

Just change over to NIC and its FREE!

; \

 

[batty]

You need to read the instructions for the meter they are all different.

The Megger cm500 I have only has two wires when doing loop tests low current and high current its the way its designed. The readings are not that accurate but its away at Megger Marks at the moment hopefully being repaired. As said read the instructions as meters work in different ways.

 

[PC Electrics]

You don't say which meter you are using, but if I were using my 1552 to carry out that test, it would require 3 leads.

 

[Lenny07]

sorry, its the megger mft 1553 95% sure anyway.

 

[Andy™]

megger 1551/2/3 need 3 leads for non-trip/ 2 leads for high current

there are some available that only need 2 lead

 

[springcrocus]

The way I understand it, three wires are needed for a no-trip test for the following reason.

To stop the RCD tripping, the sensing coil in the RCD needs to be saturated, or magnetised, with a DC voltage. Since most RCD's do not have an earth connection, the meter applies the DC across the the L and N for a given period. It then performs the measurement between L and E before the magnetic field decays.

If you only use two leads (L and E), how would you saturate the coil, given that there is no E connection?

 

[ADS]

The way I understand it, three wires are needed for a no-trip test for the following reason.To stop the RCD tripping, the sensing coil in the RCD needs to be saturated, or magnetised, with a DC voltage. Since most RCD's do not have an earth connection, the meter applies the DC across the the L and N for a given period. It then performs the measurement between L and E before the magnetic field decays.

If you only use two leads (L and E), how would you saturate the coil, given that there is no E connection?

Some testers use this 'saturation' method, but I think the 'Meggers' use the 'low current' method to prevent tripping - i.e. 15 mA test current. (the normal test uses 20A)

This is why you don't get the same degree of accuracy on the 'no-trip' test.

From what I can gather, the use of three leads increases the accuracy of the reading (a resolution of 0.01 ohms).

Meggers offering 'two lead no-trip' tend to only have an accuracy resolution of 0.1 ohms.

I might be wrong, but that's how I understood it with 'Meggers'.

 

[speedster]

Hi, The Megger MFT 1553 needs three leads connected for a 'soft' test. This applies to all types of supply, i.e. TNCS TT or TNS

The very latest Megger( just on the market) does the soft test using only two leads. Saw a demo on Friday.

Speedster.