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.

 

[sambotc]

Hi Tennisball,

Page 11 at the bottom of the green OSG. I'm fairly new to this all as well, but my interpretation was 200ohms, 21ohms being something different that I am not sure about (hence question above)

Basically anything TT needs an RCD to meet disconnection times, although a lot of TT have 100ma RCDs supplied by DNO so where do they sit in the reg's with disconnection times? I presumed it was the same as 30ma? (0.4sec)

 

[tennis ball]

Yep sorry was just refering to the OP original post



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?

I found the text you are referring to on page 11 osg.

Yes it is confusing. The way I understand it is we are supposed to install an earth electrode at the consumers location with as low an impedance as possible. This would obviously be much more difficult retrospectively.

But the impedance measured between the consumers earth electrode and the dno's transformer must not exceed 21 ohms.

Has anyone ever installed a TT earth rod? How do you test the earth path between transformer and the consumers transformer?

 

[tennis ball]

Hi Tennisball,

Page 11 at the bottom of the green OSG. I'm fairly new to this all as well, but my interpretation was 200ohms, 21ohms being something different that I am not sure about (hence question above)

Basically anything TT needs an RCD to meet disconnection times, although a lot of TT have 100ma RCDs supplied by DNO so where do they sit in the reg's with disconnection times? I presumed it was the same as 30ma? (0.4sec)

This is a giant can of worms if I ever seen one!!!

 

[telectrix]

the 100mA RCDs installed at the front end of a TT installation were to achieve disconnection times in the event of a fault. the max. Zs for these devices is 500 ohms ( see table 41.5 )

 

[sambotc]

Thanks telectrix, providing disconnection time is met, it is OK to leave 100ma as method of ADS, providing installation does not require 30ma due to installation methods (cables in wall >50mm etc), right? Or have I missed something ?


Also how do you interpretate the 21ohm statement re TT installations in OSG, what exactly are they refering to?

 

[brucelee]

Yep sorry was just refering to the OP original post



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?

the earthing conductor would not be connected to the sheath of a tns system and would not be connected to the neutral in the Head it would run of to a earth electrode somewhere in garden
the 21 ohms is the suppliers side at the earth rod at the transformer end
200ohms is the max from consumers earth electrode to the suppliers
but as Tel say try to get a reading around 100ohms or less and the rcd 30mA is for if you fail to meet the max Zs for the circuit from RA X by i delta n ie 30mA rcd which is 0.03A is less than or equal to 50v
Ra is the sum of reistances of the earth electrode and the protective conductor(s) connecting it to the exposed conductive part

to find the max of any rcd its Ra divided by I delta n Ie 30ma
50 divided by 0.03A gives you 1666.666667 which when rounded up gives you the max Zs of 1667ohms
a 100mA rcd
50 divided by 0.1 = 500ohms
a 300mA RCD
50 divided by 0.3 = 167 ohms rounded up
a 500mA rcd
50 divided by 0.5 = 100 ohms max

idelta n is the rated residual operating current
if it was a TN system then its
Zs xIa is less than or equal to Uo
Ia is the current in amperes causing the automatic operation of disconnecting device within the time specified by the relevent table 41.1 was pg 46 in the BRB now pg 53 BGB

tables 41.2 are for fuses 0.4 disconnection time 41.3 is for Circuit breakers 0.4secs 41.4 are for fuses with 5secs disconnection time
table 41.5 is for rcds and rcbos

A TT system has a max of 200 ohms at consumers earth electrode then it uses General mass Of earth to reach the suppliers Earth elaectrode which they guarantee will not be more than 21 ohms
then the suppliers earth electrode is connected to the star point of the transformer
TN-S uses the sheath and is seperate in consumers side and suppliers side and has max of 0.8
TN-C-S has a seperate earth in consumers side but then connected to the neutral in the suppliers head and from the neutral and earth are combined using a protective earthed Neutral (Pen conductor) all the way to star point and there will be earth electrodes along the way ie PME Protective Multiple Earthing
hope that makes sense and hopefully i have it right as last time i said all that was for me 2391 exam june 2010 lol