Caravan hook ups

[Doomed]

Testing a touring site for the first time - a.n.other has been doing it for years.
Incoming TNCS to glorified shed in field, outputs to hook ups earthed by SWA sheath, not connected to the big metal dis board, terminated in plastic boxes and connected to a rod so is theoretically TT'ed.
However, the readings I was getting at the hook ups seemed too good to be true, so after disconnecting everything from the rod I measured the resistance between the rod and the earth in the dis board (the TNCS earth). Result was 25 ohms.
As I said, the rod was disconnected from ever thing, so logically the resistance was through the ground.

Is this OK?

Also, elsewhere there is a poor connection in the armour between 2 hook ups, not too bad but notable. Is it worth putting another rod at this point.

 

[davesparks]

What Ra did you get for the rods and what test method did you use?

 

[richy3333]

Testing a touring site for the first time - a.n.other has been doing it for years.
Incoming TNCS to glorified shed in field, outputs to hook ups earthed by SWA sheath, not connected to the big metal dis board, terminated in plastic boxes and connected to a rod so is theoretically TT'ed.
However, the readings I was getting at the hook ups seemed too good to be true, so after disconnecting everything from the rod I measured the resistance between the rod and the earth in the dis board (the TNCS earth). Result was 25 ohms.
As I said, the rod was disconnected from ever thing, so logically the resistance was through the ground.

Is this OK?

Also, elsewhere there is a poor connection in the armour between 2 hook ups, not too bad but notable. Is it worth putting another rod at this point.

I don't understnad what you're saying. Are you saying the TNCS Earth was 25 ohms?

why put a rod in for a poor connection between 2 hook up points? Don't the connections need improving/fixing?

 

[davesparks]

I don't understnad what you're saying. Are you saying the TNCS Earth was 25 ohms?

why put a rod in for a poor connection between 2 hook up points? Don't the connections need improving/fixing?

The rods are there because they are caravan hookups.

 

[richy3333]

I understand that it's a TT earthing system, but I dont understand the scenario described? If the earth between 2 hookups(for example the SWA isn't terminated correctly causing a 'poor connection') why stick a rod in?

 

[Doomed]

I understand that it's a TT earthing system, but I dont understand the scenario described? If the earth between 2 hookups(for example the SWA isn't terminated correctly causing a 'poor connection') why stick a rod in?

'cos its corroded badley and I cant get anymore slack on them to make them off properly, ideally it would be easiest to leave that junction alone and sick a rod in.

On part 1 its 25 ohms between the source TT rod and the TNCS earth

 

[TJ Anderson]

I understand that it's a TT earthing system, but I dont understand the scenario described? If the earth between 2 hookups(for example the SWA isn't terminated correctly causing a 'poor connection') why stick a rod in?

Because you cannot use TN-CS for caravan sites so you have to convert to TT

 

[Doomed]

What Ra did you get for the rods and what test method did you use?

loop impedance test, everything disconnected from rod, 4 ohms, hence my worrys

 

[TJ Anderson]

loop impedance test, everything disconnected from rod, 4 ohms, hence my worrys

Don't worry that's great. Engineer 54 will be well proud of you mate, if it ain't single figure Ra it ain't worth having

 

[Leesparkykent]

I don't understand TBH, You are worried about having a good RA?

 

[Doomed]

I don't understand TBH, You are worried about having a good RA?

Yes, and the fact that there only seems to be a resistance of 25ohms between the earth rod and the TNCS eathing

 

[spark 68]

Yes, and the fact that there only seems to be a resistance of 25ohms between the earth rod and the TNCS eathing

I am wondering if you might possibly have a N-E fault on the TT side somewhere, this would give you that type of reading, might be worth checking, although as everything should be RCD'd on this side anyway this might not be likely.

Remember the N is effectively joined to the TNC-S E via the CNE link at the origin.

Just a thought.

 

[Engineer54]

If the earth rod has been completely disconnected from the system and you are measuring a value of 4 ohms between the Line and SWA earth on a loop test, i'd be certainly wondering where that 4 ohms is actually coming from??

How many caravan pitches is this single earth electrode position covering?? Personally i'd have a rod position at each hook-up, you're almost guaranteed a decent working TT system then...

 

[davesparks]

loop impedance test, everything disconnected from rod, 4 ohms, hence my worrys

Test the rod properly and see what you get.

 

[Doomed]

The worst Zs reading I have had so far is aroung the 5ohm area, which seems too good to be true.
System is 5 sets of 16mm SWA feeding between 6 to 8 hookup points, with each point having 4 outlets with their own RCD's and MCB's at 10 & 16.

 

[SimpleSimon]

So you have 5 separate feeds supplying 6-8 hook up points? Which means that some feeds serve more hook up points then others.

And all are protected just ONE earth rod or one earth rod per feed?

 

[Doomed]

Just the one rod, but there must be other routes to ground as the readings i'm getting are better at the outlets then the Ra reading.

Went through the clients records today, 2 other companies have signed it off over the last 8 years, its tested annually.

 

[SimpleSimon]

Maybe check to see if there are any earth pits hidden somewhere. Or if there has been extensions to building blocks since the last company came in. Maybe something is hidden. Sounds too good to be true with one rod.

 

[Engineer54]

Just the one rod, but there must be other routes to ground as the readings i'm getting are better at the outlets then the Ra reading.

Went through the clients records today, 2 other companies have signed it off over the last 8 years, its tested annually.

Something is clearly wrong somewhere, there shouldn't be any other routes to ground/earth unless they are made connections. Might be worth conducting some sheath tests if you can't find the source of this 4 to 5 ohms, not that i can see sheath damage (even to every cable) giving you those kind of values. lol!!

 

[GEOFF S]

Testing a touring site for the first time - a.n.other has been doing it for years.
Incoming TNCS to glorified shed in field, outputs to hook ups earthed by SWA sheath, not connected to the big metal dis board, terminated in plastic boxes and connected to a rod so is theoretically TT'ed.
However, the readings I was getting at the hook ups seemed too good to be true, so after disconnecting everything from the rod I measured the resistance between the rod and the earth in the dis board (the TNCS earth). Result was 25 ohms.
As I said, the rod was disconnected from ever thing, so logically the resistance was through the ground.

Is this OK?

Also, elsewhere there is a poor connection in the armour between 2 hook ups, not too bad but notable. Is it worth putting another rod at this point.

Firstly, you dont test between the rod and the earth.

Cheers

 

[TJ Anderson]

Firstly, you dont test between the rod and the earth.

Cheers

Did you used to be on SF Forum a few years back?? Mr S

 

[GEOFF S]

Did you used to be on SF Forum a few years back?? Mr S

No, ive only visited one other forum and not a regular, a newbie really.

Cheers

 

[TJ Anderson]

No, ive only visited one other forum and not a regular, a newbie really.

Cheers

I suppose it was 7 or 8 years ago now. There was a particular member on there, extremely funny IMO, but he always signed off every post with "Cheers". lol

 

[Engineer54]

I might have been miss reading this situation, if you are loop testing this single rod with everything disconnected and seeing a Ra value of 4 to 5 ohms, then that's fine. I was under the impression that you were seeing this low Ra value when testing at the hook-up points, with the rod disconnected.

As for testing between the TNC-S and the rod, not sure why you tested this, but all sorts of reasons for this value to be seen, one remote possible reason is the local DNO TNC-S multiple N-E link earth rod location. It could well be quite close-by, especially if the DNO's supply to this caravan park is derived from an overhead supply...

 

[Doomed]

I might have been miss reading this situation, if you are loop testing this single rod with everything disconnected and seeing a Ra value of 4 to 5 ohms, then that's fine. Yep, thats how it is. I was under the impression that you were seeing this low Ra value when testing at the hook-up points, with the rod disconnected. No, these readings were around the same value give or take a few ohms

As for testing between the TNC-S and the rod, not sure why you tested this, I was checking that there was a physical seperation between them, I took a reading between them first before stripping lots of stuff out of the way to confirm visually that there was no connection. but all sorts of reasons for this value to be seen, one remote possible reason is the local DNO TNC-S multiple N-E link earth rod location. It could well be quite close-by, especially if the DNO's supply to this caravan park is derived from an overhead supply...

Thank you

 

[Doomed]

Firstly, you dont test between the rod and the earth.

Cheers

Normally I wouldn't, but as the readings I was getting seemed too good to be true I was measuring to see if there was a connection between the TNCS earthing and the TT earthing before I stripped loads of stuff down to confirm a physical disconnection.

 

[GEOFF S]

Normally I wouldn't, but as the readings I was getting seemed too good to be true I was measuring to see if there was a connection between the TNCS earthing and the TT earthing before I stripped loads of stuff down to confirm a physical disconnection.

Okay, that makes sense now. Well this is one of those times when a really low RA can be problematic in the sense of PME diverted neutral currents.

Have you disconnected the swa earths then tested one by on to see if a particular ciircuit is low?

Cheers

 

[Doomed]

Okay, that makes sense now. Well this is one of those times when a really low RA can be problematic in the sense of PME diverted neutral currents.

Have you disconnected the swa earths then tested one by on to see if a particular ciircuit is low?

Cheers

That would be difficult in this situation, and as I could visually confirm the separation I have left it. I would of liked to of investigated further but it would of been an academic exercise to satisfy my curiosity, and time constraints did not allow this.

 

[GEOFF S]

That would be difficult in this situation, and as I could visually confirm the separation I have left it. I would of liked to of investigated further but it would of been an academic exercise to satisfy my curiosity, and time constraints did not allow this.

Well if the armour is the cpc, i was thinking there could be a risk of corrosion to exposed armouring/glands.

Cheers