RCBO not tripping with Hi current ELI test

[bonzer]

Had a strange situation today; I installed a couple of Hager 30ma RCBO's on a ring and radial. When I tested the Zs using the non-trip setting of Megger MFT1502, I got a high reading (7.3ohm). This was very strange considering the Ze was 0.04 and the R1+R2 about 0.56. I then tried testing using the hi current setting and it didn't trip the RCD but gave me a more realistic 0.56ohm reading. The RCD tests were fine. Why would the hi current test not trip the RCBO and why the high reading on the non-trip setting?

 

[PC Electrics]

Was this happening on one RCBO or both?

Try checking your connections, esp on the neutral side. Did you cut the neutral fly lead short to fit and not use a cord end terminal? Which could result in the fine wires spreading and making a poor (high resistance) contact.

Have you tried your tester out on another circuit (eg at home) and was it okay?

 

[SPECIAL LOCATION]

Had a strange situation today; I installed a couple of Hager 30ma RCBO's on a ring and radial. When I tested the Zs using the non-trip setting of Megger MFT1502, I got a high reading (7.3ohm). This was very strange considering the Ze was 0.04 and the R1+R2 about 0.56. I then tried testing using the hi current setting and it didn't trip the RCD but gave me a more realistic 0.56ohm reading. The RCD tests were fine. Why would the hi current test not trip the RCBO and why the high reading on the non-trip setting?

I assume you have proved that the RCBO is tripping correctly?

i.e. times and if possible trip current with a ramp test?

 

[bonzer]

yes, rcbo working fine - 28ms @ x1 and tripping @ 23mA on ramp test. its happening on both rcbos.

 

[sparkytim]

At what position are you testing.

I had similar problem with Eton Mem RCBOs trying to test at RCBO terminals.

No Trip. Test downstream and ok.

 

[bonzer]

i was testing from a socket downstream.

 

[PC Electrics]

What was the PSCC?

Is it TN-C-S or TN-S?

Have you checked resistances across the switch contacts (main sw and RCBOs)?

What results do you get if you run the two tests on a different circuit on the same installation?

Have you tried out your tester at home?

Were you using the lead with the plug?

What happens if you use the three individual leads instead?

 

[bonzer]

What was the PSCC?Is it TN-C-S or TN-S?

Have you checked resistances across the switch contacts (main sw and RCBOs)?

What results do you get if you run the two tests on a different circuit on the same installation?

Have you tried out your tester at home?

Were you using the lead with the plug?

What happens if you use the three individual leads instead?

The PSCC at the board I was working on was 15.8KA

It was TN-S

I didn't check the resistances of the switch contacts

I got the same results on the two seperate RCD circuits that had been put in.

I haven't tried the tester at home as I don't have any RCD's on my old wiring system.

I was using the plug and lead for the Megger MFT1502

I tried the plug lead with the seperate banana plugs on the end and it still didn't trip the RCD.

Incidently, before I left, I checked the Zs using the non-trip setting again and got 0.48ohms, so perhaps the tester itself is faulty. Ironically, I was borrowing a friend's Megger because my Metrel was giving me suspect readings (not unusual I hear you say!).

 

[PC Electrics]

The PSCC at the board I was working on was 15.8KAIt was TN-S

I didn't check the resistances of the switch contacts

I got the same results on the two seperate RCD circuits that had been put in.

But what about on OTHER circuits in that installation - for comparison purposes

I haven't tried the tester at home as I don't have any RCD's on my old wiring system.

You can still use the non trip test function on a socket without an RCD. Trying it out on another installation will give you readings you can compare with the suspect set.

I was using the plug and lead for the Megger MFT1502

I tried the plug lead with the seperate banana plugs on the end and it still didn't trip the RCD.

Do you mean item # 6220-782 from here http://www.megger.com/uk/webshop/index.php ?

Incidently, before I left, I checked the Zs using the non-trip setting again and got 0.48ohms, so perhaps the tester itself is faulty. Ironically, I was borrowing a friend's Megger because my Metrel was giving me suspect readings (not unusual I hear you say!)

At the same socket? or a different one (even the other one on a 2G)?

Did you operate the socket switch at all - old switches often have high resistances across the switch contacts and I've seen it on [cheap] new sockets too.

Did you reset the tester (switch off and back on) between tests?

.

Were the batteries okay (has the battery symbol come on at all during testing)?

Did the "interferance" symbol come on during the test?

Did you test and inspect the new circuits for polarity and were they okay?

 

[a1spark]

What about dodgy leads ?

a1Spark.