Box ground causes rcd to trip at > 1amp

[Yerffoeg]

I have just checked a double outlet and it trips at 18ma. It was supposed to be faulty. Washing machine had a bad heater which got replaced but using the same socket it kept tripping the rcd. Socket next to it worked okay.
Replace double socket with a new one. At the same time ran an earth wire to the metal box.
Retested. 1 ohm earth and rcd trip current now 1140ma. Removed box ground wire and rcd trip point back to 18ma. Same as the other double socket.

Incoming has earth to neutral.

Screws in faulty outlet were not very tight.

On removal the two faceplate screws were black.

Socket had 4 pairs terminated in it??

Any suggestions of where else to look?

 

[westward10]

What do you mean by 1 ohm earth?

 

[Yerffoeg]

What do you mean by 1 ohm earth?

Earth loop impedance.
This a UK installation.
About 40 years old.

 

[westward10]

Have you done an R2 to the socket earth terminal when it is isolated from the back box?

 

[Yerffoeg]

Have you done an R2 to the socket earth terminal when it is isolated from the back box?

1Mohm and 11vac.

 

[Yerffoeg]

1Mohm and 11vac.

Neutral to ground is the same 11Vac.

 

[Yerffoeg]

Neutral to ground is the same 11Vac.

Looks like a bad earth connection somewhere on the way back to the incoming?

 

[westward10]

1Mohm and 11vac.

I don't actually know what that means. Your R2 is 1Mohm?
What is 11v AC representing?

 

[Yerffoeg]

I don't actually know what that means. Your R2 is 1Mohm?
What is 11v AC representing?

Earth pin on socket to neutral or back box measures 11Vac.
I don't have the kit with me to do an r2 measurement.
The resistance of the earth pin on the socket to the metal back box is 1Mohm.
I have recommended that the house owner gets an electrician in to inspect all the socket outlets and test the entire system. It has not had an inspection for over 30years.

 

[Yerffoeg]

So no one has any suggestion on what might be the cause of the problem?
If it help I have been using one of the isotech testers for making the measurements.