TT System

[Andy Johnson]

Hello my name is Andy the NIC EIC help line is closed today so i wondered if anyone could help me please. 
I have been working on a barn conversion in South Devon and installed a consumer unit with 30mA RCBO's on every circuit with a 100mA RCD as the main switch and an earth electrode about 4 meters away driven into the ground. 
The earth electrode resistance is very high at the moment some 596 ohms!!! the 100mA RCD will not trip out on times 1 but trips out on times 5 at 16ms 
When testing one of the three ring circuits the Zs is 64 ohms x1 trips out fine but on x5 the main RCD trips before the RCBO. 
Q1 If the earth electrode resistance is reduced will the RCD trip on x1? 
Q2 Why is Zs at the sockets so low when RA is so high as main bonding on gas and water pipes are plastic? 
Many thanks Andy Johnson.

 

[leesparkykent]

Are the RCBO’s SPSN/double pole?  Is the upfront RCD time delayed? 

 

[Andy™]

why use a 100mA main switch if every circuit is protected at 30ma? its not going to be a great deal of use. your 596 is also too high for for a 100mA RCD as touch voltage will be >50v. your Zdb will need to be <500 for that (although you didnt say what your Zdb is). what electrode have you used? you may need additional electrodes either more spaced apart, longer or a mix of both. anything over 200 should be considered unstable so id be looking to reduce it more, even though as is, its acceptable for a 30mA RCD

id expect the 100mA to be a time delay - maybe you havent set your tester correctly so its either not putting a test current long enough, or the RCD does trip but in a greater time that the tester is measuring. you also dont test a 100ma at 5x, only 1/2 & 1

you dont state your Zs, there are many parallel paths. any internal pipework that go into the ground and is extraneous tht youve bonded, any structural steel buried in the ground that youve bonded etc. not just gas & water

 

[Andy Johnson]

why use a 100mA main switch if every circuit is protected at 30ma? its not going to be a great deal of use. your 596 is also too high for for a 100mA RCD as touch voltage will be >50v. your Zdb will need to be <500 for that (although you didnt say what your Zdb is). what electrode have you used? you may need additional electrodes either more spaced apart, longer or a mix of both. anything over 200 should be considered unstable so id be looking to reduce it more, even though as is, its acceptable for a 30mA RCD

id expect the 100mA to be a time delay - maybe you havent set your tester correctly so its either not putting a test current long enough, or the RCD does trip but in a greater time that the tester is measuring. you also dont test a 100ma at 5x, only 1/2 & 1

you dont state your Zs, there are many parallel paths. any internal pipework that go into the ground and is extraneous tht youve bonded, any structural steel buried in the ground that youve bonded etc. not just gas & water

Hi Andy

The 100 mA RCD (not timed delayed) was selected for discrimination 

the RA is high because of ground type shilety hope the spelling is right , the electrode was driven down about 1200mm 

The 100 mA RCD (not timed delayed) was selected for discrimination 

the RA is high because of ground type shilety hope the spelling is right , the electrode was driven down about 1200mm 

 

[Andy Johnson]

Sorry can't seem to reply on here

 

[Andy Johnson]

Are the RCBO’s SPSN/double pole?  Is the upfront RCD time delayed? 

Hi Lee the RCD is not timed delayed!!

 

[leesparkykent]

The 100mA is offering no discrimination at all. The RCBO’s are going to almost definitely be solid neutral so in the event of a neutral-earth fault the RCBO will trip, the neutral doesn’t disconnect so the 100mA RCD trips as well. Even changing it to a time delayed will result in the same outcome so unless the RCBO’s are single pole switched neutral or double pole then the 100mA is going to be a nuisance because of the lack of discrimination. Because the board is all RCBO then you don’t really need the 100mA anyway because there aren’t any flexible tails linking  RCDS like you get in a dual RCD board that require earth fault protection etc. Have you used a rod that can be coupled together and be driven deeper? 

 

[Andy Johnson]

Hi Lee thanks for your response , Weston power would not provide a main earth so i had to drive an earth rod down into the ground .the soil is very shillety ground so i cannot get the impedance down at the moment.

I was told by a senior friend to change the main disconnector to a 100mA RCD ???

 

[leesparkykent]

Hi Lee thanks for your response , Weston power would not provide a main earth so i had to drive an earth rod down into the ground .the soil is very shillety ground so i cannot get the impedance down at the moment.

I was told by a senior friend to change the main disconnector to a 100mA RCD ???

The 100mA is a waste of money IMO. The safe touch voltage given by the big book is 50V. 50V/100mA=500 Ohms and you’ve got a Ra of over 500 Ohms. The RCBO’s are still providing the appropriate earth fault protection as 50V/30mA=1666.6 recurring. I would try extra rods and link them together to try and achieve a better Ra....At least one that is considered stable. 

 

[Andy™]

you cannot achieve discrimination on RCD's by using a higher mA rating. you still havent answered my question of why you are using it anyway if everything is on a 30mA

instead of relying on 'i was told by xxx to do yyy', how about you read BS7671 and check what you are told is correct instead of blindly following wrong information?

different areas have different ground conditions - im lucky. usually a single 1mtr rod will be very low, usually <50. other areas you will need multiple rods of many lengths screwed together or other electrodes such as buried tape & many others

 

[kerching]

I was told by a senior friend to change the main disconnector to a 100mA RCD ???

Id like him to explain why that would work

just saying

 

[Doc Hudson]

If you have a fault current of say, 250ma (approx. 57.5 watts) and a 100ma RCD connected in series with a 30ma RCD, as 250ma is greater than both the 30ma AND the 100ma, it will be purely down to which RCD has the fastest operating characteristics as to which device operates first. You have NO discrimination in your design unless you have a time delayed device connected nearest to the incoming supply.

Or to put it another way; A 30ma RCD, must operate somewhere between 15ma and 30ma, A 100ma RCD must operate somewhere between 50ma and 100ma. So any earth leakage faults from 15ma (approx. 3.45watts) up to 49ma (approx. 11.27 watts), will be operating the 30ma RCD only. Any fault currents of 50ma (approx. 11.5watts) or greater, it will be a pot-luck race as to which RCD operates first.  Who designed this installation? it sounds like a very poor design to me. 

Doc H. 

 

[Doc Hudson]

I was told by a senior friend to change the main disconnector to a 100mA RCD ???

Who is signing the electrical installation certificate, (Design/Install/Test), Your friend or you?   If it is you, and you don't understand some aspect of a job, I would advise you NEVER just take the word of anybody, unless they can explain to you the science or regulations behind the design concepts of the work you are doing.

Doc H.

 

[Andy™]

I was told by a senior friend to change the main disconnector to a 100mA RCD ???

Id like him to explain why that would work

just saying

many 'electricians' falsely think itll protect the tails...

 

[binky]

1 potential solution to your problem is compact double pole RCBOs available via SBS Dave, one of our fourm sponsors. 

I know the ground you desrcribe well as I live in Plymouth and work a lot around Salcombe. You will just have to try using extendable rods, earth tape, possible multiple rods, changing locations etc etc - PITA but you don't have much choice unless you can access a steel pillar that has been driven into the ground? Given the amount of rain we have had lately, your current rod will be about as much use as a chocolate fireguard come a dry spell of weather.

 

[kerching]

many 'WANNABE electricians' falsely think itll protect the tails...

Fixed that for you

 

[Sidney]

More rods or use tape or even consider conductive concrete. Or all three.

 

[Andy Johnson]

Thank you all .