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.

 

[LastManOnline]

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.

When replacement of metallic services with plastic really started to take off , it compelled the DNO,s to review their supply system type.At the time I hoped to cash in on this change by alerting homeowners in advance to the potential consequences for their installations and advise on what changes ,if any, they would need to make.
Firts I needed to contact with the DNO,s and the providers of services (water and gas etc) to establish what their plan of action was .The result ?.There appeared to be no plan and little or no co-ordination between the various services.It was all very haphazard.Services were replaced. homeowners would start to get "tittles" from the sink (or worse) and the electrician is then called in.
I could,nt establish which services would be making replacements in which locations.
Aside from a business opportunity gone out the window,a great opportunity for a reliable earthing system was lost.
Much of the replaced metalwork was actually left in the ground and could have been reused, providing a perfect earth rod for a TT system and a great protection against an open PEN in a TN-CS system.

 

[pc1966]

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

I would just add that disconnecting the service pipe bonds does not guarantee the installation's CPC is isolated. Very likely you well find they are still linked via an immersion heater CPC, boiler CPC, gas hob ignition CPC, etc.

 

[Rockingit]

If you think about it.....your installation might be TT, but the building down the road may well be TNCS. If an incoming 'common' water pipe servicing the road is metal (cast iron!) then that same pipe is also being EQPZ bonded internally down the road - straight on to the DNO's earth. Hence how you can get really low readings.

 

[LastManOnline]

If you think about it.....your installation might be TT, but the building down the road may well be TNCS. If an incoming 'common' water pipe servicing the road is metal (cast iron!) then that same pipe is also being EQPZ bonded internally down the road - straight on to the DNO's earth. Hence how you can get really low readings.

Key point that.It means you could walk away from your TT installation and not appreciate that you are vulnerable to your TNCS neighbours issues.
An open PEN could result in the neighbouring houses current trying to get back to the DNO,s trafo via any available cpc in your house.