TT system testing - problem

[geoffa]

Hi, can anybody throw some light on a problem / confusion i have?
I'm in the process of installing a new TT system.
The existing earth electrode had a high resistance in excess of 2Mohms. I've inserted a new electrode in a new location and measured the resistance immediately after inserting the electrode with a Fluke 1653 MF tester which gave a reading of 140 ohms.
I also replaced the earthing conductor from the DB to the new electrode with 4mm 6491X, and replaced the main protective bonding conductors to the services with 10mm 6491X.
All ok so far i thought.
I've installed a new 4mm radial cct and the Zs reading came out at 39.6 ohms.
I expected the reading to be higher than 140 ohms. For it to be so much lower than the electrode reading i assume that the soil conditions have have caused a better conduction from the electrode during the week that it had been in the ground, even though no rain had fallen.
This is where i started getting confused as i then conducted the RCD tests. Results were as follows:
0deg at x0.5 = 310mS
180deg at 0.5 = 310mS
180deg at x1 = 310mS (this should be less than 200 mS as i understand it)
0deg at x1 = 310mS (this should be less than 200 mS as i understand it)
0deg at x5 = 50mS (this should be less than 40 mS as i understand it)
180deg at x5 = 50mS (this should be less than 40 mS as i understand it)
The RCD didn't trip for any of the tests.
I also measured the Idelta N value of the RCD and had a reading of 33mA.
Could someone explain why i got these results please.
And, would i be correct in saying that to meet the standards set out in the reg's this is acceptable so long as supplementary bonding is added to the system.
I think it would be far safer if TT systems were abandoned altogether and PME systems were a mandatory requirement. Zs would be a constant un-changing and reliable value and you have the obvious safety advantage of ADS using the MCB's together with the RCD protection.
Thanks in anticipation.

 

[IQ Electrical]

What is the stated I delta n of the RCD?

Is it a 'front end' RCD?

Is it an 'S' type time delayed RCD? (looks likely).



Assuming it's not an 'S' type then maximum tripping times for a BS 61008/61009 device are 300mS at 1X and 40mS at 5X.

The stated TT disconnection time of 0.2 seconds is for a fault of negligible impedance (see note 2 below the table of disconnection times).

 

[geoffa]

It's not a time delayed RCD and it's 30mA rated

 

[ratpig]

Your lower Zs reading is more likely caused by a parallel earth path from a gas or water bond on the property. This is not unusual.

 

[IQ Electrical]

It's not a time delayed RCD and it's 30mA rated

Your Zs reading will often be better than your Ra reading because of parallel earth paths once your main protective bonding conductors are connected.

I take it that where you've indicated 310mS, your meter displayed >310mS and the device did not activate?

I'm puzzled here as it does display the characteristics of a time delayed RCD?

Failing that, it could be a faulty RCD.

Did you try setting the meter for a time delayed test?

 

[geoffa]

Probably right i hadn't disconnected main bonding conductors when measuring Zs.
Any idea why i'm getting excessive times on testing RCD?

 

[geoffa]

Sorry i'm not sure if it was 310mS or greater than 310mS.
It's a new RCD but may be faulty.
No i haven't carried out a time delayed test. What's the difference between that and the other....

 

[IQ Electrical]

The test is applied for a longer duration before displaying the >? message.

 

[Des 56]

The readings are typical of that type of installation
The rcd charecteristics or the testing procedure may need checking

PME systems have latent dangerous situations that TT systems do not have
PME systems were a money saving system used when times needed it
Neither system is perfect,each has its pros and cons

 

[diditrain]

Just a thought, were all loads disconnected and did you perform your tests purely at the RCD?

 

[Dunc]

Probably right i hadn't disconnected main bonding conductors when measuring Zs.

Just to make a point that I hope you already know, Zs readings are always carried out with all earthing conductors connected at the consumer unit. The only live tests carried out with any earthing conductors disconnected is the Ze test which applies to the main earth only. Just felt I had to add this as your previous comment could be confusing for others.

 

[Engineer54]

The readings are typical of that type of installation
The rcd charecteristics or the testing procedure may need checking

PME systems have latent dangerous situations that TT systems do not have
PME systems were a money saving system used when times needed it
Neither system is perfect,each has its pros and cons

Think you'll find that there isn't very many DNO true TN-S systems left these days. Every joint that is now made or repaired on the DNO's systems and networks, the DNO'S are roding the neutral, no matter what the system earth is....

I agree with the OP on this one, all these TT systems, old and new should be converted to PME. Agree that none of the earthing systems are perfect including TN-S, but the least preferable system on a domestic installation is by far a TT system....

 

[geoffa]

Thanks for your comments.
Went back to site today and re-tested.
had some strange stuff going on.
Dropped out the main bonding conductor to start.
Tested RCD #1 which had 1 lighting cct on and 1 20A radial feeding 1 single socket.
Results were as follows:
X.5 at 0deg = >310
X.5 at 180deg = >310
X1 at 180deg = 23.3mS
X1 at 0deg = 33mS
x5 at 0deg = 11mS
X5 at 180deg = 5.1mS
IdeltaN = 24mA
Happy with that.

RCD#2 which has 32A radial for socket circuit (which i had original problems with) was then tested also at the board.
Results were as follows:
X.5 at 0deg = >310
X.5 at 180deg = >310
X1 at 180deg = >310
X1 at 0deg = >310
x5 at 0deg = 50mS
X5 at 180deg = 50mS (while doing this test RCD#1 tripped !?)

Checked that the consumer had been wired correctly at factory which it was. Also checked my connections to make sure circuit conductors were not crossed in anyway on Neutral bars which they weren't. The light circuit MCB was switched off and the socket on the 20A radial wasn't being used when the test was conducted so

So howcome the wrong RCD tripped out?

I swapped the 20A radial circuit conductors to an MCB fed from RCD#2 instead and re-tested.

RCD#2 tested again at a socket in the 32A radial circuit.
Results were as follows:
X.5 at 0deg = >310
X.5 at 180deg = >310
X1 at 180deg = 35.4mS
X1 at 0deg = 25.6mS
x5 at 0deg = 12.3mS
X5 at 180deg = 22.7mS
IdeltaN = 24mA
Good results but nothing had changed on that circuit.
Only suspect thing i was aware of was once when i reset RCD#2 and i tested it at the board i heard the RCD click attempting to trip but the switch didn't throw itself off.
I'll change the RCD as it is faulty and prob the reason also for the strange readings prior to getting good ones.
Doesn't explain why the opposite RCD tripped out though when testing RCD#2.
Any suggestions would be welcomed.
Thanks