Some Advice On Testing If you will.........

[tennis ball]

Guys,

A few things I experienced this week.

First one was where I had installed a solar installation. Basically completed the install fired up the inverter and was promptly givena red light indicating an "isolation fault." This was on a Samil River Solar inverter which ur company is just trialling. Anyway, after testing I found my ZS was 0.4 ohms, the system was PME.

I thought this was a bit high so I investigated and eventually found no earth connection on the circuit I had instaled, the termination wasn't made. What I cannot understand though is how I was able to obtain a ZS reading at all.

The instrument is a new an in calibration megger 1720 with no malfunctions and the batteries are only two weeks old.

On a proteus isolator you have a back to front terminal and you cannot see the termination, its effectively blind. Anyway the earth was not made and it just pulled straight out when I tugged at it.

QUESTION: How the hell did I get a ZS reading?!!

Its baffled me.

Anyway next question/problem.

On a TT system on Friday I measured a ZE on a TT system of 2.79 ohms. R1+R2 was 0.28 giving a ZS of 3.07 ohms, I didn't actually measure the ZS but just added R1+R2

Looking at my regs book now I can see the max ZS for a 16A type b rcbo is 2.87 ohms so I am outside of this.

Without performing a calc to see wether 4mm cable could have brought down the resistance of my R1+R2 and ZS within limits, I have a more demanding question,

If TT ZE is allowed up to 21 ohms, how do you meet maximum ZS stated in regs for a TT system?




I appreciate any feedback,

Dan

 

[telectrix]

max ZS for a 16A type b rcbo is 2.87 ohms so I am outside of this.

max. allowable Zs for any 30mA RCD/RCBO is 1667 ohms

 

[tennis ball]

Ha!

The true value of the 30ma RCD/RCBO rears its head!! You really do learn something new everyday.

Always thought table 41.5 had something to do with earth rod resistance or some other unknown quantity.

Thanks very much!

 

[Flyingsparks]

Guys,

A few things I experienced this week.

First one was where I had installed a solar installation. Basically completed the install fired up the inverter and was promptly givena red light indicating an "isolation fault." This was on a Samil River Solar inverter which ur company is just trialling. Anyway, after testing I found my ZS was 0.4 ohms, the system was PME.

I thought this was a bit high so I investigated and eventually found no earth connection on the circuit I had instaled, the termination wasn't made. What I cannot understand though is how I was able to obtain a ZS reading at all.

The instrument is a new an in calibration megger 1720 with no malfunctions and the batteries are only two weeks old.

On a proteus isolator you have a back to front terminal and you cannot see the termination, its effectively blind. Anyway the earth was not made and it just pulled straight out when I tugged at it.

QUESTION: How the hell did I get a ZS reading?!!

Were you testing phase & neutral rather than phase & earth ?

 

[tennis ball]

Unfortunately, I do not know. I was having difficulty in getting a reading so maybe I connected all three!! I had a cold and was under pressure to get the job finished and it was a nightmare to tell the truth so anything could have happened.

But as I guess you are hopefully going to tell me, its possible to get a ZS through the Neutral on a PME?

 

[tennis ball]

max ZS for a 16A type b rcbo is 2.87 ohms so I am outside of this.

max. allowable Zs for any 30mA RCD/RCBO is 1667 ohms

This does lead me to another question though.

If I was just installing MCB's and there was no requirement for an RCD and the zs was outside the guidance of the regs, what then? I guess you would just employ the most cost effective solution to meet the regs but would be keen to hear what anyone has to say on that?

 

[sambotc]

You would have got a reading for PSCC rather than PEFC I am guessing?

 

[Flyingsparks]

On a PME system the Zs reading should be the same as the reading between phase & neutral.

You may have had your tester set to phase - neutral test

 

[sambotc]

Isn't max Ze on TT 200ohms?

 

[telectrix]

thats a recommended figure for Ze anything over that may well be unstable, but a figure of around 100 i9s more than often not obtained. the 1667 is max. Zs for as circuit protected by RCD, as on a TT system, you rely on the RCD for ADS.because the MCB/Fuse will not break within the specified time limit.

 

[sambotc]

Yep sorry was just refering to the OP original post

If TT ZE is allowed up to 21 ohms, how do you meet maximum ZS stated in regs for a TT system?




I appreciate any feedback,

Dan

Not quite sure what the OSG means by 21ohms being the usual max stated resistance of the distributors earth electrode at the supply transformer, is this something you would see out on a domestic job as I don't think I know what a TT with distributors earth before if thats the case? How would you tell, would there be an earth following the incoming tails from overhead lines or similar?

 

[tennis ball]

Isn't max Ze on TT 200ohms?

I always thought it was 0.8 tns, 0.35 tncs and 21 for TT? Funny thing I cannot find this anywhere. However if you look on page 50, table 41.5 NOTE 2: * The resistance of the installation earth electrode should be as low as is practicable. A value exceeding 200 ohms may not be stable, refer to regulation 542.2.2

Which Says:

Basically it accounts for variable environmental conditions such as drought and freezing.

Can anyone point out where max ZE values for various earthing systems can be found?

 

[Guest 004]

Unfortunately, I do not know. I was having difficulty in getting a reading so maybe I connected all three!! I had a cold and was under pressure to get the job finished and it was a nightmare to tell the truth so anything could have happened.

But as I guess you are hopefully going to tell me, its possible to get a ZS through the Neutral on a PME?

If you got the reading from the neutral, it would have been lower

 

[tennis ball]

On the (only TT system I have tested) there was an overhead line with two separate conductors. No earth. Then I followed the earth from the Distribution Board and found the rod.

What page is that in the osg?

 

[telectrix]

you can't meet the max. Zs values for MCBs on a TT system. so you fit RCD/RCBO. so the max. Zs is 1667 ( to nearest whole no.) and it should be noted that the max. disconnection time of an RCD in a TT system is 200mSec. at x1.

edit this value is derived from 50/0.03, where 50V is an acceptable touch voltage, and 0.03 is the tripping current In of the RCD.