TT Zs Readings and Tripping RCD?

[paulk11]

Afternoon. Did a PIR on a TT installation yesterday with some confusing results.

Ze/RA was recorded as 116Ohms so as expected thereabouts. When Zs was tested it was found to be just 1.01 Ohms and 0.97 Ohms at a several points with just a few reading high as expected. As Zs = Ze+(R1+R2), it seems inconceivable that it can be that low. I',m thinking an alternative earth has sneaked in somewhere; (possibly a link to a TN system somewhere nearby?)

Following the Zs test, the RCD (S Type) started tripping and also failed the RCD test. We're now in a position where the RCD will stay made for a while but keeps tripping out for no apparent reason.

My initial thought is to change the RCD and see how that effects things.

Any suggestions?

 

[telectrix]

your Zs readings are probably due to the extraneous water and gas supply pipes, which are a far better earth than a thin 3ft. stick in the ground. as for the RCD, no idea why. it could be faulty and having tripped once is now unstable. or you may have inadvertently introduced a fault on the installation if you have dismantled and reassembled sockets or otherwise.

 

[rich.250]

The low Zs readings are more than likely a service bond.
It's not unheard of for a near by property to be tncs and the service pipe is acting as the conductor going back to the suppliers earth in the other property.

As for the RCD, it was probably stuck. And now you have tested it is tripping.
If it failed the test it needs replacing.
If your insulation resistance readings are ok and no faults present, then that would be a good place to start.

 

[paulk11]

OK thanks a lot. There is no gas and the water bonding doesn't exist as it's a plastic incomer from the supply tap outside. The internal building domestic water system supplementary bonding is in place however, so I presume that's the only route the earth would have via the copper pipework. IR was all sound although was down to 125MegOhms - whole system tested in a single shot.

I'll get out and purchase a new one I think and give that a go for starters.

Cheers.

 

[paulk11]

The RCD will hold for a while but then eventually trips for no obvious reason - it's not an instantaneous reaction on making the RCD switch.

One other thought, would the RCD be tripping if there was an imbalance of earths - i.e. a rod combined with a.n. other source? The RCD would respond to a difference in L-N?

 

[Deleted member 26818]

A neutral to earth fault would produce low Zs readings on a TT system.
Would also cause the RCD to trip once someone had fun freed it up conducting tests.

 

[Octopus]

The RCD will hold for a while but then eventually trips for no obvious reason - it's not an instantaneous reaction on making the RCD switch.

One other thought, would the RCD be tripping if there was an imbalance of earths - i.e. a rod combined with a.n. other source? The RCD would respond to a difference in L-N?

I think you need to read up on how RCD's work.

Is it a 30mA RCD or 100mA?

 

[paulk11]

I think you need to read up on how RCD's work.

Is it a 30mA RCD or 100mA?

Thanks for that Murdoch; think I have it all clear - toroidal coils etc! Take the question more as seeking confirmation of a firm comprehension of the operation also, my text above confirms the reason why an RCD would trip.

30mA delayed

I did get some very useful info from fellow posters and having just replaced the RCD in question all seems good now with this particular RCD however it seems there may be a problem with another one so no wonder we were baffled by this inspection.

 

[Octopus]

30mA delayed

I did get some very useful info from fellow posters and having just replaced the RCD in question all seems good now with this particular RCD however it seems there may be a problem with another one so no wonder we were baffled by this inspection.

??????

 

[paulk11]

Yes..... It's a RCCB on DB2. Will change it this week if the client wants and hope that's an end to it. All good fun.

 

[Octopus]

So have you got 2 RCD's in series?

 

[paulk11]

Set up is:
DB1: DP Iso switch with various circuits split between 2 30mA delayed RCDs so discrimination is achieved.
DB2: 30mA RCCB protecting 3 circuits.
Both DBs are supplied separately from mains supply.

 

[Octopus]

Set up is:
DB1: DP Iso switch with various circuits split between 2 30mA delayed RCDs so discrimination is achieved.
DB2: 30mA RCCB protecting 3 circuits.
Both DBs are supplied separately from mains supply.

why the tine delay if they are final circuits?

 

[paulk11]

It's what we found when we turned up to do the inspection so not planning to change things. I'll be recommending that the RCCB gets changed and all will be good........

Thanks for the advice - it threw us yesterday but hopefully sorted soon.

 

[Octopus]

It's what we found when we turned up to do the inspection so not planning to change things. I'll be recommending that the RCCB gets changed and all will be good........

Thanks for the advice - it threw us yesterday but hopefully sorted soon.

this all sounds very odd.

could you post a picture of each CU? Not seen one with 2 x time delayed rcd's

 

[R G]

The other thing is with a seized RCD is there could well be faults present but now the RCD has been released although faulty , would now mean that simply replacing it would also mean rectifying the faults too..

 

[PEG]

Only thing i could add to above,is, has leakage been clamped? and was test apparatus suitable for testing TD RCD?

 

[tony mc]

Why would you put a time delayed 30 mA units in as they are not offering additional protection for anything as they will not disconnect in time to offer this or is there something special about this instal we need to know about.

I might be missing the obvious here as I am not that bright!

 

[Sintra]

Didn't even know a 30mA time delayed RCD or RCBO existed.

 

[davesparks]

Didn't even know a 30mA time delayed RCD or RCBO existed.

They do, but I have only ever known them be made to order by specialist protection companies. I can't imagine them ever being used in anything other than a specialist installation, they aren't exactly cheap!

 

[Sintra]

They do, but I have only ever known them be made to order by specialist protection companies. I can't imagine them ever being used in anything other than a specialist installation, they aren't exactly cheap!

Yes indeed. I've had a quick search and such beasts can be ordered but surely putting these into a domestic situation for final circuits would mean that the tripping times required of 300mS @ 30mA and 40mS @ 150mA are not met.

 

[paulk11]

OK, all credibility (as if I ever had any anyway) is about to be ruined!

Having checked the info, they are standard RCDs and not time delayed!! I was going on colleagues info (honest) and not physically checked the details myself. So all advice above is much appreciated and fully understood. It's all a learning process :sick:

Cheers

 

[davesparks]

Yes indeed. I've had a quick search and such beasts can be ordered but surely putting these into a domestic situation for final circuits would mean that the tripping times required of 300mS @ 30mA and 40mS @ 150mA are not met.

Yes exactly, they are no use for shock protection.
I think they see use as a backup fault protection when 10mA protection is used. But I could be wrong

 

[Deleted member 26818]

Can be used for shock protection, not for additional protection.