Earth Loop Impedance Testing

Bus bar · 1 Sep 2015

In the USA we typically use a 120/240 volts 3 wire supply with a neutral for our residential mains conductors.
Example , they are run mostly overhead from a pole mounted utility transformer. There is only one wire feeding the primary side of the utility transformer located 300' away from the meter socket mounted on the side the house. We use the same basic type of earthing connection as UK for new construction. We do no Loop impedance testing here at all which is hard to believe in this day and age. I'm firmiliar with your testing schemes and was wondering if Ze test would read too high of an impedance through the earth 300' to the xtrm ??
Also we typically drive two 8'rods six feet apart with hopes of arriving at 25 ohms or less no test here either?? I hope to use a fluke 1650 model and was hoping some of you bright UK sparkies could share some on how to go about doing this test correctly?!

JohnW2 · 1 Sep 2015

Bus bar said:
Also we typically drive two 8'rods six feet apart with hopes of arriving at 25 ohms or less no test here either?? I hope to use a fluke 1650 model and was hoping some of you bright UK sparkies could share some on how to go about doing this test correctly?!

There really is not a lot of point in measuring loop impedance when the installation's earth is by local rod(s) (a 'TT' earthing system), since it will inevitably be too high for fuses/MCBs to provide adequate protection against live/line-earth faults - in such a situation, one has to rely on an RCD for that protection.

Kind Regards, John

Bus bar · 1 Sep 2015

JohnW2 said:
Bus bar said:

Also we typically drive two 8'rods six feet apart with hopes of arriving at 25 ohms or less no test here either?? I hope to use a fluke 1650 model and was hoping some of you bright UK sparkies could share some on how to go about doing this test correctly?!

Click to expand...

There really is not a lot of point in measuring loop impedance when the installation's earth is by local rod(s) (a 'TT' earthing system), since it will inevitably be too high for fuses/MCBs to provide adequate protection against live/line-earth faults - in such a situation, one has to rely on an RCD for that protection.

Kind Regards, John

I'm good with all you say above
Our supply mains reads 240 volts between L1 & L2 and 120 volt to neutral or ground(earth) they are 3 separate conductors . Two hot wires and an insulated neutral. The two hot wires are I believe 90 degrees out out phase?Still considered single phase? Since they are not 120 degrees apart to be called two phase??We connect our ground rod wire to the main neutral that is connected to terminal bar in the meter socket would this still be equivelent to TT system?
All of our over head installation for residential customers consist of this type triplex wire
We run the 4th insulated earth wire only after the main breaker to a sub panel and separate the ground wires (earth wire)from neutrals.
We could still do Zs testing??
Everything here now is arc fault breaker protected. Do they use in UK yet?

endecotp · 1 Sep 2015

180 degrees apart.

JohnW2 · 1 Sep 2015

Bus bar said:
I'm good with all you say above. Our supply mains reads 240 volts between L1 & L2 and 120 volt to neutral or ground(earth) they are 3 separate conductors . Two hot wires and an insulated neutral. The two hot wires are I believe 90 degrees out out phase?

I would have thought 180°.

Bus bar said:
We connect our ground rod wire to the main neutral that is connected to terminal bar in the meter socket would this still be equivalent to TT system?

Are you saying that your ground rod(s) are connected to the incoming neutral? Do you know if that neutral is earthed at the transformer?

Kind Regards, John

Bus bar · 1 Sep 2015

We do use GFCI outlet(sockets) and breakers. What you call RCD in the UK

I would like see Zs testing done here in the USA since we have so many prolems with loose wiring connections , call back and fires as a result. I think you have a really good system for testing in the UK and I know it is performed in most country around the world except here in th US.

Bus bar · 1 Sep 2015

JohnW2 said:
Bus bar said:

I'm good with all you say above. Our supply mains reads 240 volts between L1 & L2 and 120 volt to neutral or ground(earth) they are 3 separate conductors . Two hot wires and an insulated neutral. The two hot wires are I believe 90 degrees out out phase?

Click to expand...

I would have thought 180°.

Bus bar said:

We connect our ground rod wire to the main neutral that is connected to terminal bar in the meter socket would this still be equivalent to TT system?

Click to expand...

Are you saying that your ground rod(s) are connected to the incoming neutral? Do you know if that neutral is earthed at the transformer?

Kind Regards, John

It could be 90 + 90 degrees for a total 180

Ground rods are connected to the incoming neutral correct
Separate Ground rod also connects to transformer at the pole that is correct?
TT?

Bus bar · 1 Sep 2015

How many conductors do you have in your TN-NS
Is it one L1 , one neutral,and earth all the way back to the trans former

Eg is it One combined Concentric cable SWA with core neutral and outer armour used as main earth back to transformer

Thank you for you replys

OwainDIYer · 1 Sep 2015

Bus bar said:
Everything here now is arc fault breaker protected. Do they use in UK yet?

No. We don't use wirenuts or pigtails either. These facts may be related.

PBC_1966 · 1 Sep 2015

The standard arrangement in the U.S. is equivalent to British TN-C-S, with the neutral of the transformer secondary connected to earth locally and a rod at each service entrance connected to the incoming neutral. The connection must be made at or before the service-disconnecting means, which in practice means that it's usually connected either to the neutral busbar in the main distribution panel or at the meter base.

Bus bar said:
Example , they are run mostly overhead from a pole mounted utility transformer. There is only one wire feeding the primary side of the utility transformer located 300' away from the meter socket mounted on the side the house.

This can vary from place to place. Many utility companies distribute the HV neutral and have local transformer primaries connected phase to neutral, but there can also be phase-to-phase connections for the single-phase transformers used to feed regular residential neighborhoods. Some places do not distribute the HV neutral at all, so all local primaries are phase-to-phase connections, as in the U.K.

We use the same basic type of earthing connection as UK for new construction.

There are several types of earthing arrangement in the U.K. though, TN-C-S resembling the American system. The TT arrangement which is common in Britain is not permitted under the NEC.

wondering if Ze test would read too high of an impedance through the earth 300' to the xtrm ??

Not if the service conductors are sized properly. It's not at all unusual for the 240/415V distribution in the U.K. to run for much longer distances than that, since British practice tends toward installing one large transformer and running a 4-wire 3-phase network around a whole neighborhood, whereas American practice leans more to using a larger number of smaller single-phase transformers, each feeding only a few homes (although this is changing gradually in some newer subdivisions).

Also we typically drive two 8'rods six feet apart with hopes of arriving at 25 ohms or less no test here either??

Remember that the NEC specifies the use of two rods only if a single rod cannot be brought below 25 ohms by itself. It is curious though that if 25 ohms cannot be achieved with a single rod, the rule then just says to install a second rod without any further stipulation!

Bus bar · 1 Sep 2015

OwainDIYer said:
Bus bar said:

Everything here now is arc fault breaker protected. Do they use in UK yet?

Click to expand...

No. We don't use wirenuts or pigtails either. These facts may be related.

I h
I Know you use terminal strips but not sure if your regs allow some type of splice inside switch boxes ?Of course ,the more wire splicess pig tails , or terminal strips the more likely the chance of having a high impedance connection somewhere

PBC_1966 · 1 Sep 2015

I Know you use terminal strips but not sure if your regs allow some type of splice inside switch boxes ?

Yes, they do.

Bus bar · 1 Sep 2015

PBC_1966 said:
The standard arrangement in the U.S. is equivalent to British TN-C-S, with the neutral of the transformer secondary connected to earth locally and a rod at each service entrance connected to the incoming neutral. The connection must be made at or before the service-disconnecting means, which in practice means that it's usually connected either to the neutral busbar in the main distribution panel or at the meter base.

Bus bar said:

Example , they are run mostly overhead from a pole mounted utility transformer. There is only one wire feeding the primary side of the utility transformer located 300' away from the meter socket mounted on the side the house.

Click to expand...

This can vary from place to place. Many utility companies distribute the HV neutral and have local transformer primaries connected phase to neutral, but there can also be phase-to-phase connections for the single-phase transformers used to feed regular residential neighborhoods. Some places do not distribute the HV neutral at all, so all local primaries are phase-to-phase connections, as in the U.K.

We use the same basic type of earthing connection as UK for new construction.

Click to expand...

There are several types of earthing arrangement in the U.K. though, TN-C-S resembling the American system. The TT arrangement which is common in Britain is not permitted under the NEC.

What type or system are we using if it's not TT?

wondering if Ze test would read too high of an impedance through the earth 300' to the xtrm ??

Click to expand...

Not if the service conductors are sized properly.

We don't install the ground wire needed(earth conductor ) all the way back to the xtrm to preform Ze test this is the problem not the size of the conductors

It's not at all unusual for the 240/415V distribution in the U.K. to run for much longer distances than that, since British practice tends toward installing one large transformer and running a 4-wire 3-phase network around a whole neighborhood, whereas American practice leans more to using a larger number of smaller single-phase transformers, each feeding only a few homes (although this is changing gradually in some newer subdivisions).

Thoughts on why we don' test Zs ??if a plumber is require to pressure test his pipe surely the electrician would be required to loop test his wires for high impedance and leaks to prevent fire, after floods rather then a Hail Mary blessing!

Also we typically drive two 8'rods six feet apart with hopes of arriving at 25 ohms or less no test here either??

Click to expand...

Remember that the NEC specifies the use of two rods only if a single rod cannot be brought below 25 ohms by itself. It is curious though that if 25 ohms cannot be achieved with a single rod, the rule then just says to install a second rod without any further stipulation!

No test required by NEC to proof the single rod meets this 25 ohm requirement as I'm sure you'll agree

JohnW2 · 1 Sep 2015

Bus bar said:
JohnW2 said:

I would have thought 180°.

Click to expand...

It could be 90 + 90 degrees for a total 180

well, yes, but I wouldn't call that "90 degrees out of phase"!

Bus bar said:
Ground rods are connected to the incoming neutral correct ... Separate Ground rod also connects to transformer at the pole that is correct? TT?

No, if that's all correct, it sounds like the equivalent of the UK TN-C-S system, although we wouldn't normally add the local rods to it.

Kind Regards, John

Bus bar · 1 Sep 2015

Bus bar said:
PBC_1966 said:

The standard arrangement in the U.S. is equivalent to British TN-C-S, with the neutral of the transformer secondary connected to earth locally and a rod at each service entrance connected to the incoming neutral. The connection must be made at or before the service-disconnecting means, which in practice means that it's usually connected either to the neutral busbar in the main distribution panel or at the meter base.

Can you clarify that we are actually using TN-CS system here in the U.S.
The problem that I see is this - in the UK if I'm correct they use L1+ separate neutral & separate earth wire all the way to xtrm and drive a rod at both locations . or do they piggy back the earth wire onto the neutral for the ride back to the xtrm?
In the US. We have two hots one neutral we should have a fourth wire going all the way back since the we can't rely on a ground type TT system back through the earth. To much earth impedance to effively take out the fuse
So our system shouldbe four wire to xtrm other wise it looks like a TT system
does anyone have a drawing of UK and U.S. System to confirm all of this confusion

Bus bar said:

Example , they are run mostly overhead from a pole mounted utility transformer. There is only one wire feeding the primary side of the utility transformer located 300' away from the meter socket mounted on the side the house.

Click to expand...

This can vary from place to place. Many utility companies distribute the HV neutral and have local transformer primaries connected phase to neutral, but there can also be phase-to-phase connections for the single-phase transformers used to feed regular residential neighborhoods. Some places do not distribute the HV neutral at all, so all local primaries are phase-to-phase connections, as in the U.K.

We use the same basic type of earthing connection as UK for new construction.

Click to expand...

There are several types of earthing arrangement in the U.K. though, TN-C-S resembling the American system. The TT arrangement which is common in Britain is not permitted under the NEC.

What type or system are we using if it's not TT?

wondering if Ze test would read too high of an impedance through the earth 300' to the xtrm ??

Click to expand...

Not if the service conductors are sized properly.

We don't install the ground wire needed(earth conductor ) all the way back to the xtrm to preform Ze test this is the problem not the size of the conductors

It's not at all unusual for the 240/415V distribution in the U.K. to run for much longer distances than that, since British practice tends toward installing one large transformer and running a 4-wire 3-phase network around a whole neighborhood, whereas American practice leans more to using a larger number of smaller single-phase transformers, each feeding only a few homes (although this is changing gradually in some newer subdivisions).

Thoughts on why we don' test Zs ??if a plumber is require to pressure test his pipe surely the electrician would be required to loop test his wires for high impedance and leaks to prevent fire, after floods rather then a Hail Mary blessing!

Also we typically drive two 8'rods six feet apart with hopes of arriving at 25 ohms or less no test here either??

Click to expand...

Remember that the NEC specifies the use of two rods only if a single rod cannot be brought below 25 ohms by itself. It is curious though that if 25 ohms cannot be achieved with a single rod, the rule then just says to install a second rod without any further stipulation!

Click to expand...

No test required by NEC to proof the single rod meets this 25 ohm requirement as I'm sure you'll agree

Earth Loop Impedance Testing

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