RCBO on TNCS system. - Earthing and testing

[philrosenberg]

Hi all, was hoping you might be able to lend a hand.

First my system. I've got a TN-C-S system, there is a CU with rewirable cartridge fuses and between the CU and the meter is a 30A/30mA RCBO.

I've just fitted 3 new plug sockets and am testing everything. As I've been going down my checklist I've come up with a few questions.

First bonding, all my bonding is 6mm so was going to replace it with 10mm, however the earth cables from the CU to the load side of the RCBO and the supply side to my earthing rod are also in 6mm. Is there a safe way to open up the RCBO to change these as I don't want to open it up and find live terminals staring at me that I can't shut off.

second insulation testing. I have a BM8 Mk2 insulation tester and it is coming up with values of P/N=1000 MOhm, P/E=700MOhm and N/E=300MOhm. Do these sound right? I was expecting more like 10 MOhm and for them all to be about the same. I've checked the test leads and they are fine.

Lastly my Earth loop tester trips the RCBO. I have R1+R2 and can call the supplier tomorrow to get the impedence for their side, but I wondered if there was any safe way to bypass it that I haven't thought of.

Thanks for any help anyone can give

Phil

 

[plugwash]

First my system. I've got a TN-C-S system, there is a CU with rewirable cartridge fuses and between the CU and the meter is a 30A/30mA RCBO.

are you sure its a RCBO?

if its a RCBO then you'll trip it if you have any big loads in your installation, if its just a RCD you risk damaging it with overloads.

I've just fitted 3 new plug sockets and am testing everything. As I've been going down my checklist I've come up with a few questions.

First bonding, all my bonding is 6mm so was going to replace it with 10mm, however the earth cables from the CU to the load side of the RCBO and the supply side to my earthing rod are also in 6mm. Is there a safe way to open up the RCBO to change these as I don't want to open it up and find live terminals staring at me that I can't shut off.

you mention an earth rod, is there also an earth connection to the service cut out? or is your system actually TT?

second insulation testing. I have a BM8 Mk2 insulation tester and it is coming up with values of P/N=1000 MOhm, P/E=700MOhm and N/E=300MOhm. Do these sound right? I was expecting more like 10 MOhm and for them all to be about the same. I've checked the test leads and they are fine.

the higher the better, all those values are high enough to be of no concern at all

post some photos please so we can see what your setup is really like

 

[Adam_151]

Measure your Ze on the supply terminals of the RCD and add this to the R1+R2, this will be more accurate than asking the supplier who will just quote you 0.35

Does your loop tester have a low current test, you can use this to do the Z's on the fianl circuit

Oh, and ditto everything plugwash said... your set up sounds weird

 

[philrosenberg]

are you sure its a RCBO?

Just checked what was is written on the fron, got the first rating wrong sorry. It says "80 A I(triangle)n 0.03 A~ (Ohm)" the bits in brackets are symbols that I can't type. I was presuming that the two ratings were the two protection levels of an RCBO, but could the 80 A be a max overload rating for an RCB.

you mention an earth rod, is there also an earth connection to the service cut out? or is your system actually TT?

Again I'm making an assumption here. I cannot see for sure that the earth cable from the CU connects to the neutral tail as it's inside the RCBO and I don't want to open it up as I'm not sure if there'll be exposed live terminals inside. However , I spoke to my distributer and they said all there installations are TN-C or TN-C-S. Saying that I know the house has been rewired at least once in the past so I can't guarentee that the spark who did it didn't change it.

the higher the better, all those values are high enough to be of no concern at all

I know the norm is more than 2 MOhms, but when I got 1000, I started getting suspicious, that's all. It's like if your bank said, invest your money here and you'll get at least £2 interest per month, then when you do you actually get £1000 you might start to think a mistake has been made. But if 1000 MOhms sounds normal then I'm happy.

post some photos please so we can see what your setup is really like

Will do, give me ten minutes to work out how

Cheers

 

[JohnD]

Just checked what was is written on the fron, got the first rating wrong sorry. It says "80 A I(triangle)n 0.03 A~ (Ohm)" the bits in brackets are symbols that I can't type. I was presuming that the two ratings were the two protection levels of an RCBO, but could the 80 A be a max overload rating for an RCB.

this is an RCD, rated current 80A (this is generally enough for a house) with a 30milliamp trip. This is a bit sensitive for a whole house, it will probably nuisance-trip from time to time from normal slight leakages adding together from cooker, microwave, immersion heater, PC and washing machine.

... I've got a TN-C-S system... my earthing rod

It is very irregular for a TNC-S system to have an earth rod. How did you identify these two points? Are you working from a book? Which one?

First bonding, all my bonding is 6mm so was going to replace it with 10mm, however the earth cables from the CU to the load side of the RCBO and the supply side to my earthing rod are also in 6mm. Is there a safe way to open up the RCBO to change these as I don't want to open it up and find live terminals staring at me that I can't shut off.
Phil

The Main Earthing does not need to go through the RCD, it should go to the CU in 16mm, and to the incoming gas and water mains in 10mm. It does no harm to have an earth to the RCD enclosure, especially if it is metalclad, as some of the more modern ones have a trace earth connection.

 

[RF Lighting]

post some photos please so we can see what your setup is really like

Will do, give me ten minutes to work out how

Cheers

You will need to use an image host such as Photobucket

 

[plugwash]

are you sure its a RCBO?

Just checked what was is written on the fron, got the first rating wrong sorry. It says "80 A I(triangle)n 0.03 A~ (Ohm)" the bits in brackets are symbols that I can't type. I was presuming that the two ratings were the two protection levels of an RCBO, but could the 80 A be a max overload rating for an RCB.

sounds like a RCD indeed

you mention an earth rod, is there also an earth connection to the service cut out? or is your system actually TT?

Again I'm making an assumption here. I cannot see for sure that the earth cable from the CU connects to the neutral tail as it's inside the RCBO and I don't want to open it up as I'm not sure if there'll be exposed live terminals inside.

if its a modern unit wired properly all live terminals should be deep enough down holes that you can't touch them directly. Having said that do take care if you remove the cover just in case its been wired badly (the same goes when removing the cover of a CU thats connected to a mains supply you can't turn off).

However , I spoke to my distributer and they said all there installations are TN-C or TN-C-S.

just because a suppliers earth is availible doesn't mean its actually in use.

 

[Spark123]

Are you sure they didn't say TN-S or TN-CS?

 

[philrosenberg]

One image, just click it to open a larger version in a new window.


The wiring inside the CU is a little untidy but it's just how I found it except for one end of the ring main that I replaced to put one of the new sockets in and everything seems in order.

In the large image you can see 2 earth cables coming out of the bottom of the CU, one goes off to the gas pipe for main bonding, the other follows the P/N cables into the RCBO (look carefully, it's hidden a bit).
There is also an earth cable coming out of the bottom of the RCBO and heading off the bottom of the photo that goes outside to an earth rod.
Not entirely sure what's in the box to the left, but a big cable covered in black tarry cloth comes out the bottom and heads out of the house under the concrete floor. It has an anti-tamper screw on it so I'm assuming its the supplier's switch or something?

 

[JohnD]

It is not TNC-S as there is no G/Y earth coming out of the service head.

However if you ask the network co there may be able to convert it and provide you with PME. There may be a charge. This is the usual system in the UK.

 

[plugwash]

get the lid off that RCDs enclosure and get us another picture.

 

[philrosenberg]

Sorry edited cos I've just worked out what is going on.
I seem to have a TT system. Below is a pic of my RCD opened up, click for larger version

the earth cables on the bottom go to the electrode and the CU

 

[JohnD]

p.s. Does that Wylex CU have a metal case? there should be grommets in all the knock-outs to prevent cables chafing, I can't see them. Openings in the top need to be sealed tightly to prevent anything getting in. All others need to be a close fit round the cables.


Look on the bright side, it's rather old and not to current standards, but it's tidy and simple. Having an RCD as a main switch makes it easier and safer to work on the CU.

edited to say - the earth cables in the RCD enclosure are only parked there, when you put the new ones in you can bypass it if you like. Have a look at the earth terminals in the CU to see if they will fit a 16mm cable (I think they will as it is a reasonably late one).

And edited again - you appear to have the RCD "ON" and the CU Main Switch "ON" although you have the case of the RCD enclosure and the CU open. This is not good practice.

 

[philrosenberg]

it is metal cased yes, and you are right there are no gromets. That's going to be another fun job.

Okay, I seem to have everything sorted out as far as my system is concerned (although I'm not sure my distributer will agree with me, maybe I'll have to phone them again)

All i wanted was a few extra sockets and it's turning into a right old job.

I think I have all my questions answered now except for the earth loop testing.
Is there a safe way to do it with my tester that trips the rcd? If not then I'm a bit stumped cos the distributer didn't have any details for TT systems when I last called them.

 

[philrosenberg]

And edited again - you appear to have the RCD "ON" and the CU Main Switch "ON" although you have the case of the RCD enclosure and the CU open. This is not good practice.

Sorry you are of course right, if it makes you feel any betterI'm the only one in the house and it's only like that so I could have my pc on to send these questions and take photos.

But of course I should have closed it up before turning on the power to stick my pc on.

also I'd just like to say a big thanks to all who have posted.
I think it's time for a beer or two before last orders.