Zs read on Megger 1552

[ukants]

Is there any way to do a Zs read at the furthest point of a 6a mcb protected lighting radial without the mcb tripping? The circuit is not RCD protected. I can understand why the high current two lead test would trip it, but the non-trip three lead test also trips the mcb. Is this usual? Should I just add Ze to r1+r2 on every lighting circuit from now on?
Thanks.

 

[securespark]

It trips the MCB?

Ze is incoming earth loop, Zs is from circuit. Which are you attempting?

 

[ukants]

Hiya Securespark,
Yes it trips the MCB. I'm attempting to measure the Zs from the furthest point on my lighting circuit, to compare with the Ze+r1+r2 reads. I've tried both the hi-current two lead test (red lead to phase/ green lead to cpc) and the low current three lead test (add black lead to neutral to above connections) and both tests are tripping the MCB. Any ideas? Am I missing something very simple (it's not unknown ). Thanks.

 

[Gasman1015]

I have the same problem with my megger, depends on the length of the radial circuit, only way I can see to solve it is to buy the LTW325, guaranteed not to trip.

 

[ukants]

Ahh...thanks Gasman. Looks like calculations then!

 

[DESL]

Am I missing something very simple

Yep afraid so - megger puts a 10A load on circuit to measure Zs

 

[ukants]

Yep afraid so - megger puts a 10A load on circuit to measure Zs

Not as far as I'm aware. I'll check the data sheet but I think it sends out 25A on the high current test (so understandably trips) but only 15mA on the three-lead non-trip test.

 

[DESL]

On non trip it loads L-N up to 25A but limits the current to earth at 15mA.

Just calculate Zs

 

[Cremeegg]

May be an old thread but was chatting with Megger the other week - who are as helpful as usual. It appears their tech staff never guaranteed a no trip option 100% of the time for the 1552 but their sales people jumped on the idea. In my experience it tripped about 50% of the time before the upgrade. The MCB chattering as it struggled not to trip.

Megger will offer an upgrade to the 1552 so it does not trip a 6A MCB - well worth getting done when you next get it calibrated - if you can wait that long. Contact their Tech Dept for details.

 

[Steve3948]

Just replace the mcb with a 10A for the duration of the test then replace it after.

 

[bernardgreen]

Just replace the mcb with a 10A for the duration of the test then replace it after.

Wouldn't a 10 amp MCB have a lower impedance than a 6 amp MCB and if so would this then not affect the final test result.

 

[Steve3948]

Just replace the mcb with a 10A for the duration of the test then replace it after.

Wouldn't a 10 amp MCB have a lower impedance than a 6 amp MCB and if so would this then not affect the final test result.

Bernard, I have tried this with a 10A and 16A and the difference was very slight to nothing.

 

[GaryMo]

Just replace the mcb with a 10A for the duration of the test then replace it after.

An old thread but don't do that, just calculate Zs.
Replace MCB, phone rings, apprentice falls off steps and you forget to change the MCB back.
Calculation is an accepted way of obtaining Zs values, in fact more so with regards to section 14 of the EAWR.

 

[Steve3948]

Just replace the mcb with a 10A for the duration of the test then replace it after.

An old thread but don't do that, just calculate Zs.
Replace MCB, phone rings, apprentice falls off steps and you forget to change the MCB back.
Calculation is an accepted way of obtaining Zs values, in fact more so with regards to section 14 of the EAWR.

Whatever floats your boat