UK I am getting very high zs readings

[banny07]

Hi Guys. When measuring zs I get very high reading. Ze is 0.3 (tns), zdb is 0.21. r1 and rn are is 0.42. r2 is 0.72 , r1+ r2 is 0.29 . when I test zs it 8.26 ohm new to testing. I even buypassed rcbo but it stays same. Do you think my mft if faulty? Any thoughts guys? I am new to testing.

 

[Julie.]

None of those figures make sense to me.

I assume zdb is at the db after measuring ze so should be higher than ze not lower.

How are you doing these tests?

What is the state of the circuit for each test?

 

[westward10]

If Ze was measured with parallel paths removed then Zs at the DB with all paths reinstated can be lower.
If you believe your test equipment to be faulty then recheck it at your known source of Zs at the DB.

 

[banny07]

None of those figures make sense to me.

I assume zdb is at the db after measuring ze so should be higher than ze not lower.

How are you doing these tests?

What is the state of the circuit for each test?

I am teasting ze with main earth disconnected, zdb with earth connected.

 

[banny07]

None of those figures make sense to me.

I assume zdb is at the db after measuring ze so should be higher than ze not lower.

How are you doing these tests?

What is the state of the circuit for each test?

On lighing circuit r1+r2 is 0.86 but zs comes up to 7.66 ohm.on another radial circuit r1+r2 is 0.09 but zs jumps up to 0.52.

 

[timhoward]

Formatted slightly differently....
Please note r1, rn and r2, and R1+R2 are different things.

Ze is 0.3 (tns), (OK)
Zs@db is 0.21. (OK - parallel paths I assume)
Ring Final Circuit:
End to End r1 and rn are is 0.42. (OK)
End to End r2 is 0.72 (OK, assuming twin and earth - expected cpc= * 1.67 and is close enough)

R1+ R2 is 0.29 (please use capitals for this - OK - 0.42+0.72 / 4 = 0.28 )

. when I test zs it 8.26 ohm new to testing. NOT OK!

What make and model of MFT do you have?
Are you using different test lead / probe / clip for Zs?
If recent Megger are you selecting 2 wire or 3 wire non-trip Zs?

 

[westward10]

Have you tried a line to neutral loop. I often do this with obscure loops, one it verifies your meter is working and two it verifies the line and if this is okay you could have an earthing issue. With higher than expected loops return to the board and verify your loop at the board again it isn't uncommon for a TN-S system to intermittently break down.

 

[banny07]

Have you tried a line to neutral loop. I often do this with obscure loops, one it verifies your meter is working and two it verifies the line and if this is okay you could have an earthing issue. With higher than expected loops return to the board and verify your loop at the board again it isn't uncommon for a TN-S system to intermittently break down.

R1 and Rn are same on lighting circuit

Formatted slightly differently....
Please note r1, rn and r2, and R1+R2 are different things.


What make and model of MFT do you have?
Are you using different test lead / probe / clip for Zs?
If recent Megger are you selecting 2 wire or 3 wire non-trip Zs?

Megger 1741+ . using plug leads for sockets and using original test leads to test lighting circuit. using 2 low or 3 low leads with same result.

 

[timhoward]

Well something is afoot!
I think you need to retrace steps and do several Zs@DB in a row to check results are repeatable, and also double check main earth is properly reconnected.
If the high current loop is working consistently and reliably for the board, there is no reason it shouldn't also work reliably when testing the final circuit with RCBO bypassed.
Try (carefully!) using the exact same probes at the back of a socket to rule out a faulty plug adaptor test lead.

It's plausible that the non-trip Zs is being affected by interference/noise, but the high current should be fine.
(There is a way to use high-current with RCBO's but I won't confuse the issue by talking about that yet.)

 

[banny07]

Please note r1, rn and r2, and R1+R2 are different things.

Thanks. point noted.

 

[banny07]

It's plausible that the non-trip Zs is being affected by interference/noise, but the high current should be fine

I used 2 lead on high setting when I bypassed RCBO.

 

[Julie.]

I am teasting ze with main earth disconnected, zdb with earth connected.

Fair enough, although you should be testing for zs using main earth only, removing parallel paths.

Zs can vary a bit even from test to test, but shouldn't be that far out.

You can easily check your mft for correct zs, at home or wherever, measure the zs (say 1. 3 ohm), then measure it again but place a 1 or 2 ohm resistance between your meter leads and the same point, the new zs readings should increase by the value of the resistance you added. (To 3.3 ohm in this example) if it's all OK, them you are measuring something wrong, or you have a poor connection when you reconnected either the circuits, or the main earthing.

There is a possibility of cross voltage from another property, is the bonding correct?

 

[timhoward]

I was about to suggest similar to @Julie -
Another thought - if you have a long wander lead with you, measure it's resistance on the continuity range.
Then do Zs@DB again normally. Then Zs@DB with the wander lead between the tester and the CPC bar. It should increase by the resistance first measured.

 

[banny07]

you should be testing for zs using main earth only, removing parallel paths

I didn't know that. I thought zs db/circuit should be tested with all parallel path connected.

 

[banny07]

I was about to suggest similar to @Julie -
Another thought - if you have a long wander lead with you, measure it's resistance on the continuity range.
Then do Zs@DB again normally. Then Zs@DB with the wander lead between the tester and the CPC bar. It should increase by the resistance first measured.

I will do this when I go back there. I have another mft (1553) which I will be also using . My concern is how to proceed if It also gives same results.