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Topic Title: How to measure diverted neutral currents
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Created On: 10 July 2014 07:51 AM
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 10 July 2014 07:51 AM
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leckie

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So, off on a tangent from a previous thread

How to measure diverted neutral currents.

So in my own building, it's a 60A TPN TNCS with steel A frames extending into the ground. It joined to other similar buildings.

I measured 10mA with a clamp meter around meter tails L1,L2,L3 & N. So that is measuring the earth leakage of the installation.

I measured about 6A around the earthing conductor. So is this a measurement of diverted neutral current plus it would include the building earth leakage?

If I put a clamp meter around the 4-cores of a SWA TPN cable and measure say 10mA this would again be the earth leakage of the load that cable is supplying. So if I put a clamp meter around the outside of the swa and there were stray neutral currents within the cable armour, would this measure it?
 10 July 2014 08:56 AM
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alanblaby

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You are not at all clear as to what. and how you are measuring anything.
What load is on? Is the load constant?
Clamp meter around the tails - what all together, or individual tails?
6 amps on the earthing conductor? That is a very large amount when we'd be expecting mA.
What is the load when such a current is going down the EC?
Measuring with a clamp meter around SWA - is that possible?
 10 July 2014 09:18 AM
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leckie

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Around all the meter tails. 10mA
Just the earthing conductor 6A. It's PME and the building is steel framed, so it like a giant earth rod!

So, as I said the 10mA reading I think is the earth leakage of the installation, it can't be going anywhere else, and so I am assuming that the 6A through the earthing conductor is diverted neutral current.

I am going to do the test again in a while so I will check how similar it is.

Regarding the SWA, I'm going to try that this morning as well, so I will post the results.

Edit to add:
The load on the meter tails would have been quite small probably only 5-10 A per phase.
 10 July 2014 09:30 AM
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John Peckham

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Yes clamping all 4 of the tails will give you an approximate earth leakage for the installation. Clamping the earthing conductor will give you the current flowing in the earthing conductor. What you cannot see is the direction it is flowing from. It could be diverted currents from other premises from their bonding to you common pipework and down your earthing conductor or you could have current flowing in your bonds from pipe to pipe or pipe to structure.

Out of interest what clamp meter are you using? Is it a True RMS meter and can you change the frequency response? I ask as if you or the adjacent premises are generating harmonics then things become or lot more complicated.

Also how big are your premises? What size supply?You indicate you have a 3 phase supply and a 10mA leakage so the install cannot be that big?

PM me your email address and i will send you something useful on leakage currents I received recently.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 10 July 2014 10:19 AM
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leckie

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John

Its a 60A TPN, small commercial premises on an industrial estate, with mixed used business premises, garages, electronics manufacturers, taxi firm, etc. I don't usually have much more than some lighting and laptops on.

The low current ammeter (mA) is a Kaise SK7815. The data sheet says it has a low pass filter (cut off 150Hz or more), whatever that means in practice I am not sure. I am not sure if it is true RMS or not, it doesn't say in the instructions.

I checked the higher reading with a Martindale CM58, and that is true RMS and gave a similar reading.

Just checked again this morning and the earthing conductor current had dropped to 4.5A. Only one phase in the building was pulling any load at the time, about 7A.

The only service pipe is the water, and that's plastic. But we have lots of steel and that's bonded to the MET.

So if

Edited: 10 July 2014 at 10:26 AM by leckie
 10 July 2014 10:27 AM
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Parsley

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http://instrotech.com/download...,2M3FQ,77O5J7,9JMWR,1

Doesn't go into diverted neutral currents but might be useful. there's a bit about different frequencies.

Regards
 10 July 2014 10:39 AM
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leckie

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Thanks for that Parsley, so basically what I was saying. The Blue Book for petrol filling stations allows this method as an alternative to insulation testing in some cases.
 10 July 2014 01:00 PM
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leckie

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I have now been to a job and tried to test for diverted neutral currents on a large SWA cable. I put a clamp meter around all the lives, inc neutral, cores and it measured at 25mA. I measured around the outside of the actual SWA cable and it measured 200mA. So I am assuming that the difference i.e. 175mA is diverted neutral current?
 10 July 2014 01:35 PM
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AJJewsbury

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I have now been to a job and tried to test for diverted neutral currents on a large SWA cable. I put a clamp meter around all the lives, inc neutral, cores and it measured at 25mA. I measured around the outside of the actual SWA cable and it measured 200mA. So I am assuming that the difference i.e. 175mA is diverted neutral current?

I'm hesitant to say. Logically it should be, but I'm a bit worried about the effect of the ferrous armour when you're trying to cancel out current in the cores against current in the armour - would the magnetic field generated by the cores escape the cable sufficiently to cancel out the magnetic field generated by the armour current? If it was a supplier's CNE cable with a copper 'armour' I'd be a lot less worried. Maybe a bit of a controlled experiment would tell us - although I suspect you might be in a better position to do that than me.

- Andy.
 10 July 2014 02:12 PM
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leckie

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I will have a play and post my findings
 10 July 2014 02:25 PM
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mapj1

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So long as the iron does not form a loop to form another core inside the core formed by he jaws of the current clamp, you are quite alright, and you are OK, because the wires in the SWA are parallel to the current not surrounding it.
(and if it did, it would under read, not over read, as to a degree it would be magnetically shielding the current clamp from seeing the wires.)

The current you measure can be thought of as either

1) current that flows down the cores not matched by an equal and opposite current on the outer jacket, or

2) current that flows down the outer jacket not matched by an equal and opposite current on the cores

Sloppily folk sometimes talk about the inner currents (meaning the balanced bit ) and the outer currents, (meaning what you are seeing, the un-balanced bit).

This gives the impression that two independent currents really do flow, one on the inside and one on the outside of the armour, and at 50Hz it doesn't really.

But in practice for your purpose, the effect is the same, the un-balanced difference current must get on or off by a different route, presumably via the extended earth electrode formed by the building and also via cross-bonded service pipes that serve many buildings and then via the jackets of other cables that supply those buildings.

The 150Hz roll off effectively means that the meter will spread out short duration events into a lower value of longer duration, so simplistically a 10A 1millisecond spike, may read the same as a 1A 10ms one. In terms of talking about heating etc, its not perfect RMS but is still much better than the more common peak hold type of meter, which can't tell a 10A spike from a 10A continuous signal and says '10' in both cases.

-------------------------
regards Mike
 11 July 2014 10:48 AM
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perspicacious

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Turn off the 4 pole main switch and then clamp the earthing conductor.

Regards

BOD
 11 July 2014 09:01 PM
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leckie

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Well the one in my unit is three pole. But I will do that anyway and see what happens! You clever man
 15 July 2014 03:25 PM
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AJJewsbury

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Turn off the 4 pole main switch and then clamp the earthing conductor.

That'll account for diverted N originating from loads in neighbouring installations, but will obviously exclude any N from your own loads - which might be significant...
- Andy.
 15 July 2014 08:24 PM
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perspicacious

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"How to measure diverted neutral currents."

Turn off the 4 pole main switch and then clamp the earthing conductor.

"That'll account for diverted N originating from loads in neighbouring installations, but will obviously exclude any N from your own loads - which might be significant..."

I read this as what is diverted into leckie's premises from the adjacent buildings. When leckie has an unbalanced three phase with his own neutral current, it would be interesting to see Mr Kirchoff's contribution

Regards

BOD
 15 July 2014 09:05 PM
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mapj1

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If he has PME, and some single phase or even balanced three phase but 'spiky' current waveforms that result in neutral current then in reality that current will share between the proper Neutral path and any metallic services that also go to other buildings with PME & main bonding that are nearer to the substation, in the ratio of the resistance of the two paths. Given the very low impedance of metal pipes, due to their enormouse cross-section they may actually take more of the neutral current than the company PEN conductor in some installations.

-------------------------
regards Mike
 15 July 2014 09:21 PM
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leckie

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It is pme and I think the diverted currents are coming fom the outside into the building. It's 3-phase, but mainly used as a small office environment.

I'm off for a few day to watch blokes hitting a little white ball across grass into a hole in the ground in Liverpool. But I will do a few more checks next week
 15 July 2014 09:25 PM
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perspicacious

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"Given the very low impedance of metal pipes, due to their enormouse cross-section they may actually take more of the neutral current than the company PEN conductor in some installations."

I've yet to find that, as ultimately the diverted neutral current is the path from the electrodes of the installation (such as steel in ground) back "up" through the DNO electrode nearest the "star point" of the Tx which may be as much as 10 or 20 Ohms.

Who is the big mouse?

Regards

BOD
 15 July 2014 09:30 PM
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slittle

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Originally posted by: perspicacious

"Given the very low impedance of metal pipes, due to their enormouse cross-section they may actually take more of the neutral current than the company PEN conductor in some installations."



I've yet to find that, as ultimately the diverted neutral current is the path from the electrodes of the installation (such as steel in ground) back "up" through the DNO electrode nearest the "star point" of the Tx which may be as much as 10 or 20 Ohms.



Who is the big mouse?



Regards



BOD




ME !!!!
 15 July 2014 09:48 PM
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mapj1

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err OK, so after a bier or two I can't spell in a hurry. For comic value I won't go back and correct it.

But actually, the last high current 'up' point to the star point of the TX need not be an earth electrode at all, it can be the PEN to service pipe bond at the nearest property t the TX that has a PME supply - which may be the first house right next to the transformer building on a housing estate.
It is not always the case that any terra-firma current need actually flow -it can be all flow in the service pipes that run up the road parallel to the PEN and bond to it at each house with a PME supply, effectively being a parallel path to some of it, and that would remain true even if one could (as thought experiment only of course) turn all the soil into a perfect insulator.

-------------------------
regards Mike
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