Economy 7 supply

tony9i9i9i · Jun 7, 2024

Hi all, as I said before I came back to the trade last year. I've been on a couple of jobs recently where the no longer used, cancelled, "cut off" Economy 7 supply was not exactly dead, in both cases testing just over 12V across the tails.
Has anyone else noticed this?

littlespark · Jun 7, 2024

No one “cuts off” the e7…. It comes from the same supply cable and only switches on through the time clock or teleswitch.

You should never leave any tails unconnected, or empty terminal exposed to touch… just in case a fault in the timeclock or switch accidentally turns on.

davesparks · Jun 7, 2024

Can you clarify what you mean by a cut off e7 supply?

What were you t sting between and what type of tester?

Taylortwocities · Jun 8, 2024

tony9i9i9i said:
in both cases testing just over 12V across the tails.

If you were using a regular digital multimeter then you will see spurious voltages like this. Those tester things are very high impedance and will pick up stray voltages because of inductance effects between connected cables.
Hang any sort of load (eg shunt reseistor) between the two conductors and you will probably see the PD will drop to zero.

Hopefully you have a calibrated electrician's MFT?. These have a larger impedance that cancels out these ghost voltages.

tony9i9i9i · Jun 10, 2024

Taylortwocities said:
If you were using a regular digital multimeter then you will see spurious voltages like this. Those tester things are very high impedance and will pick up stray voltages because of inductance effects between connected cables.
Hang any sort of load (eg shunt reseistor) between the two conductors and you will probably see the PD will drop to zero.

Hopefully you have a calibrated electrician's MFT?. These have a larger impedance that cancels out these ghost voltages.

Hi Taylor, I've got a MFT X1, the last one was where I'd connected a pannel heater to a spur unit, the spur was off of the old E7 pannel. I did a 3 wire voltage test at the spur unit and got 12V, wouldn't run the heater obviously, but not quite dead.

davesparks · Jun 10, 2024

tony9i9i9i said:
Hi Taylor, I've got a MFT X1, the last one was where I'd connected a pannel heater to a spur unit, the spur was off of the old E7 pannel. I did a 3 wire voltage test at the spur unit and got 12V, wouldn't run the heater obviously, but not quite dead.

A 3 wire voltage test isn't something I've come across before, how does that work, does it give you 3 readings as if it was taking 3x voltage measurements simultaneously?

Check for voltage with an AVI and see what that says.

davesparks · Jun 10, 2024

Taylortwocities said:
Hopefully you have a calibrated electrician's MFT?. These have a larger impedance that cancels out these ghost voltages.

A lower impedance is required to collapse the ghost voltage and I'm not sure whether MFTs actually have such a function.

An AVI should always be able to distinguish a ghost voltage from any other.

tony9i9i9i · Jun 12, 2024

I've taken a couple of photos to explain see attached. So with probes or a plug adaptor lead, on my MFT if I connect Red and Green and test voltage I've just got L-PE 236V, L-N 214V and N-PE 48V, but add blue and its L-PE 237V, L-N 237V, N-PE 0V.
Did the same on the spur unit connected to the old E7 CU and was getting just over 12 volts, seen it twice now, and I'd be interested to know why.

newfutile · Jun 12, 2024

Is it not just telling you there is no neutral connection present ?
There are probably voltage dividers internally , my kewtech generally reads about half the supply voltage if the neutral or cpc is not connected

davesparks · Jun 12, 2024

tony9i9i9i said:
I've taken a couple of photos to explain see attached. So with probes or a plug adaptor lead, on my MFT if I connect Red and Green and test voltage I've just got L-PE 236V, L-N 214V and N-PE 48V, but add blue and its L-PE 237V, L-N 237V, N-PE 0V.

Yes, with just the red and green terminals connected you are only measuring between those two terminals. The tester displays the the voltage measured as being between the live and earth because those are the labels assigned to those terminals.
The 214V and 48V displayed will be the voltages that are present on the internal resistances in the MFT which are used in the measuring circuit and bear no relation to anything outside of the MFT.

Inside the MFT there will be a network of resistors connected between the three terminals which are used to make the voltage measurement, when only two of the three terminals are connected to a source of voltage the third terminal will float at a value proportional to those resistances.

tony9i9i9i said:
Did the same on the spur unit connected to the old E7 CU and was getting just over 12 volts, seen it twice now, and I'd be interested to know why.

As above test it with an AVI and see what you get, it is likely to be a capacitively coupled voltage (ghost voltage) which will not show up as a live voltage with an AVI.

tony9i9i9i · Jun 12, 2024

davesparks said:
Yes, with just the red and green terminals connected you are only measuring between those two terminals. The tester displays the the voltage measured as being between the live and earth because those are the labels assigned to those terminals.
The 214V and 48V displayed will be the voltages that are present on the internal resistances in the MFT which are used in the measuring circuit and bear no relation to anything outside of the MFT.

Inside the MFT there will be a network of resistors connected between the three terminals which are used to make the voltage measurement, when only two of the three terminals are connected to a source of voltage the third terminal will float at a value proportional to those resistances.

As above test it with an AVI and see what you get, it is likely to be a capacitively coupled voltage (ghost voltage) which will not show up as a live voltage with an AVI.

Thanks, guys for explaining why I get the strange readings without using three prongs. I want to say I tested with my TIS 849 as well, but I'm not certain I did. I won't get back to that job again, but will look out for another shall we say ceased E7 supply and see if I'm seeing voltage.