TT System with TNS reading

[IAmSparkytus!]

Shop with no earth coming from head, overhead lines in sight everywhere - definitely no earth from the DNO.

Two 10mm earths going into the DB. One going to the water and another which disappears through a wall somewhere heading towards an external area never to be seen again.

Customer doesn’t know where the earth rod is located (if there is one).

Took an earth loop reading at the board and tester shows 0.41ohms with water bonding disconnected.

How??!!

Work required at the store but don’t want to agree unless this reading can be verified as legitimate somehow or the customer agrees to a new rod being installed.

 

[westward10]

What loop do you get to the other 10.0 when disconnected. Just ditching the water may not remove all parallel paths.

 

[IAmSparkytus!]

Sorry 0.41 was with the tester clamped onto the 10mm earth only.

Could this earth go to another water/gas pipe elsewhere perhaps? Would you normally get such a low reading from a service pipe?

 

[westward10]

Until you can confirm where it terminates it is impossible to say.

 

[IAmSparkytus!]

never had a situation like this before so would the best bet be to leave the earth connected as we don’t know what it’s for and add in an earth rod to ensure we are always reliably connected to earth from a known source?

 

[mainline]

Usually the more, the better.

 

[LastManOnline]

Would you normally get such a low reading from a service pipe?

yes.quite common on older metallic services.

Should you leave it in place?Why not.See it as a bonus.

 

[IAmSparkytus!]

thought so but then I suppose if TTing it, fault protection will have to be put in place otherwise disconnection times wouldn’t be met so that’s another thing, would you rely on the low reading and not add RCDs or can it not be relied on as it can’t be verified as to what it is?

 

[LastManOnline]

would you rely on the low reading and not add RCDs or can it not be relied on as it can’t be verified as to what it is?

Seeing it as a bonus is as good as it gets.The old metallic service will likely at some point be replaced.
Installing rcd,s is essential.

 

[pc1966]

As above, a low Zs when bonded is usually the result of a service pipe, either it is massively low Ra or more likely at some other installation it is bonded to the same sub-station's earth system.

However, you cannot depend on that! You have absolutely no control over its presence and any replacement pipes could be plastic, etc, and render it useless.

So unless you have a supply earth (TN system) you must have a rod* and design for the TT characteristics. That means you know the rod's Ra value, it is low enough to be stable (the 200 ohms in the regs) and you have a means of ADS to match, which realistically means an appropriate RCD incomer (e.g. 100mA selective) and/or all-RCBOs, etc, so every final circuit is adequately protected should you only have the rod as a means of earthing. A good low impedance bond is simply a bonus in most case.

However, there are a few special cases where it has to be TT and not TN-C-S due to the slight risk of an open-PEN fault (caravan power, etc), in those cases you would not be allowed to bond to something that is linked to the PEN.

[*] Or other approved independent means of earthing.