Please explain to me

[PeterSmiles]

In my house each power circuit has its RCD and all plugs are earthed.

A few weeks ago I had ordered someone to come inspect the electrical installation. He had such an appliance, from which I don't know the name, which he pugged into each wall socket and then he could measure the resistance of the earth circuit and how many millisecond it would take for the RCD to trip.

He found out that there were a few wall plugs that didn't trip and as far as I understand, but I may have misunderstood this, those same wall sockets also hadn't sufficient earthing. It was corrected at the time by changing something in the junction boxes above the ceiling.

Now I was of the impression that the RCD and earthing circuit work separately from each other.

So my questions are.

- How is it possible that some sockets on a circuit would not trip the RCD, while others on the same circuit would.

- Why did the same sockets have insufficient earthing if it are separate safety systems.

[IMHO]

Probably bad on non-existent earth connections, or just wired wrong - none would surprise me.

Edited December 3, 2013 by IMHO

[trogers]

"How is it possible that some sockets on a circuit would not trip the RCD, while others on the same circuit would"

Those sockets could not have tripped the RCD because they were not tied into the circuit due to loose wiring at the junction boxes, that is, they didn't even carry power to any receptacle plugged into them.

[Crossy]

So my questions are.

- How is it possible that some sockets on a circuit would not trip the RCD, while others on the same circuit would.

- Why did the same sockets have insufficient earthing if it are separate safety systems.

Both are inter-related.

It is quite correct that an RCD does not need an earth to function, it measures the differential between the Live and Neutral currents and trips if this exceeds the trip current (usually 30mA in domestic installations). It assumes that any difference is going where it shouldn't (through you perhaps).

However, the RCD tester does need an earth in order to test the RCD (it needs somewhere to send the leakage current). With no earth at the outlet the RCD won't trip when tested (most testers will actually complain that there is no earth and so won't initiate the test).

The test sequence should be:

• Verify correct earth at each outlet.
• Check the RCD trip time for each circuit.

[PeterSmiles]

"How is it possible that some sockets on a circuit would not trip the RCD, while others on the same circuit would"

Those sockets could not have tripped the RCD because they were not tied into the circuit due to loose wiring at the junction boxes, that is, they didn't even carry power to any receptacle plugged into them.

The sockets definitely had power, because that was tested by myself and were also be connected to the correct breaker.

So my questions are.

- How is it possible that some sockets on a circuit would not trip the RCD, while others on the same circuit would.

- Why did the same sockets have insufficient earthing if it are separate safety systems.

Both are inter-related.

It is quite correct that an RCD does not need an earth to function, it measures the differential between the Live and Neutral currents and trips if this exceeds the trip current (usually 30mA in domestic installations). It assumes that any difference is going where it shouldn't (through you perhaps).

However, the RCD tester does need an earth in order to test the RCD (it needs somewhere to send the leakage current). With no earth at the outlet the RCD won't trip when tested (most testers will actually complain that there is no earth and so won't initiate the test).

The test sequence should be:

• Verify correct earth at each outlet.
• Check the RCD trip time for each circuit.

The appliances the guy had would test both with the push of a single button. I still don't get what was the exact issue, because with one socket they had to go up 2 or 3 times before it would function correct, as they always would connect the wrong wires.

[bankruatsteve]

Could be L and N were reversed and/or ground wasn't connected. Makes you wonder why it wasn't done right the first time but now that your good, I wouldn't worry about it.

[Forkinhades]

Could be L and N were reversed and/or ground wasn't connected.  Makes you wonder why it wasn't done right the first time but now that your good, I wouldn't worry about it.

Sounds like he has a good meter. If it was reverse polarity the meter would have picked that up as well.

It is purely a bad earth.

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