TT system, earth to neutral not tripping RCD

[It's-me]

TT system, Zs on socket circuit is 77.2 down a good bit after I added a second earth rod.

When I short neutral to earth RCD doesn't trip.

Can anyone help me out and explain please.

 

[mattg4321]

Is there any current on the neutral. Is anything in use at the time it's 'shorted'?

 

[i=p/u]

Just use tester 1x 5x

 

[Strima]

If there’s no current flowing in the circuit the RCD won’t see the fault.
And I would not recommend doing it this was as there is a risk of shock.

 

[westward10]

With TT and sometimes TN a connection between neutral and earth may not create an imbalance whereby the circuit may decide to still use the path of least resistance, neutral.

 

[It's-me]

With TT and sometimes TN a connection between neutral and earth may not create an imbalance whereby the circuit may decide to still use the path of least resistance, neutral.

That's what I thought, I'm not used to TT installations, they're not the norm where I am.
I'm just used to it popping straight away with TNCS.

Maybe a few more rods might lower it enough to trip.

Thanks

 

[It's-me]

Is there any current on the neutral. Is anything in use at the time it's 'shorted'?

Yea, there's definitely more than 30ma flowing.

 

[Pretty Mouth]

At the point you have shorted N and E, you have 2 paths back to the transformer, in parallel:

The neutral conductor, say approx 1 ohm, and
The earthing system, about 77 ohms.

To trip the 30mA RCD, gonna need >15mA straying to earth.

V = IR
V = 0.015 X 77
V between short and trans = 1.155V

I = V/R
I = 1.155 / 1
I in neutral conductor = 1.155A

So an absolute theoretical minimum of 1.170A flowing in the neutral at the short to trip the RCD (probably need a lot more in practice).


The current straying to earth might not come from that circuit, it might come from others on the same RCD. In which case the same calculation, but now the 2 paths would be:

Neutral conductor from N bar back to transformer, say about 0.1 ohm, and
N conductor of socket circuit from N bar to the short, then back to transformer via earthing system, about 78 ohms.

0.015 X 78 = 1.17
1.17 / 0.1 = 11.7A flowing in the N bar