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 Large earth currents in M.E.N. installations considered normal
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 Return to top of page · Post #: 1 · Written at 12:09:12 AM on 3 August 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6013

In the August 2104 issue of Silicon Chip, there’s an article by Leo Simpson titled “Your House Water Pipes Could Electrocute You” in which he discusses situations in the MEN wiring scheme where significant current flows in the earth circuit as a matter of course.

He cites his own house where, using a clamp meter, he found under normal circumstances up to 55% of load current was flowing as return current in his earth circuit and 45% in the neutral.

He also measured 4 amps or more at his water meter after he turned off the main switch at his meter board. (His installation uses the water pipes as the main earth connection). I don’t know how that can occur other than that he was seeing earth return current originating in nearby premises that is using his neutral as the return path?

Overall, his measured figures surprised me. I only expected a continuous high earth current flow under circumstances where the incoming supply neutral connection was bad (i.e. high resistance).

Simpson arranged for an Energy Australia electrician to inspect and measure his installation and the sparky found the resistance of the neutral path to be 0.35 ohms and the earth path to be 0.3 ohms, which apparently is within spec (AS/NZ3017). No wonder the current favoured the earth return to the neutral.

I have been aware for a long time that plumbers are supposed to bridge across a water meter with a jumper cable before they remove a meter in order to avoid possible electric shock, but I did not realise how common it is to find high current flowing through the pipe system.

My own installation, which I gather is newer than Simpson’s house, uses an earthing rod rather than the water pipes. However, I intend to put a clamp meter on the earth cable to see what leakage current is present.

I would be interested to hear what figures other members find in their own MEN installations. (Note: Don't attempt this unless you know what you are doing!)


 
 Return to top of page · Post #: 2 · Written at 12:37:53 AM on 3 August 2014.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6353

I am not sure when earth stakes became compulsory but it can be assumed that it is at least 40 years and most likely more. Water pipes and indeed other similar infrastructure cannot be used as an earthing medium for the purpose of the MEN system. What does happen is that the water pipe gets earthed via an equipotential bond to protect building occupiers from electric shock should the pipe become live. Similar equipotential bonds are tied to roof guttering, steel building frames and the reinforcing steel in concrete pools for the same reason. The actual MEN link involves all these bonds and the earthing conductors from the installation being connected to the neutral conductor in the main switchboard. MEN links should not be in place at any other part of the installation.

If there is 4 amps flowing through an occupier's water pipes then there is a serious issue to resolve or the tong tester is giving a false positive. There are often stray currents flowing through earthing systems but they should never come to 4 amps or anything close to it. If this is happening whilst the occupier's main switch is turned off and there is no faulty wiring found then the current will be feeding in from other installations, most likely from an installation close by that has lost its reference to earth altogether.

The highest current flow through a correctly earthed installation I have seen in my time as an electrician is about 280mA, due to the dozens of computers fed from it and this was in a commercial environment. A standard household shouldn't even have anywhere near this flowing through earth.

I'll clamp the main earth in the block of flats I am in tomorrow (Sunday) and report what I find.

Update: I think one of the main benefits of an earth stake these days is because in many cases the pipework (or much of it) throughout many modern buildings is plastic after the meter.


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A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 3 · Written at 1:00:40 AM on 3 August 2014.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4347

I have single phase coming in on 2 aerials and some 240V aerials still in operation. Originally there was a grid put down for a single wire SWER (1960's) and I believe part of that to still be connected to parts of the installation. On the LV side three of the posts are metal box section 1 wood, and the other 7 truss.

While 5 of the fuse / distribution boxes have their own earth spikes & one other may come back to another close box; the neutral on one meter, is on the three box section posts & connected to them: They are about 5 feet into the ground. Most of the water pipes are plastic. I have not bothered to see what current is running in the earth spikes. The best earth I had was in the previous place, where it got tied to a 125 foot metal bore casing in brackish water.

One little nasty that did not manifest (or rare) when we had metal pipes was the ability of the bath & sinks to become alive. Where there are plastic drains that often insulates a metal sink, or bath where the taps are not on the bath / sink.

I did make a point in the previous dwelling of linking the earth of a hob to the metal plate (tab welded on) and the kitchen sink (taps on wall). I had had the fuse box upgraded to current specs.

One day the electric jug decided to spring a leak, which was noted after the water manged to trickle over (laminated bench) to the sink & the RCD killed the circuit. So there is something to do a risk assessment on.

Marc


 
 Return to top of page · Post #: 4 · Written at 1:14:37 AM on 3 August 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6013

Suggest folks check out the subject Silicon Chip article, in which Simpson powers up a 2400 watt (i.e. 10A) heater and measures the neutral and earth. As pictured, he gets 4.32A in the neutral and 5.06A in the earth.

The Energy Australia sparky ran a system integrity check of his house via a Fluke 1654B Tester and pronounced his installation to be "quite safe".


 
 Return to top of page · Post #: 5 · Written at 1:20:28 AM on 3 August 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6013

If there is 4 amps flowing through an occupier's water pipes then there is a serious issue to resolve or the tong tester is giving a false positive.

That was my feeling, too, but the EA sparky's neutral and earth resistance readings on Simpson's installation would account for the return current favouring the earth path in the proportion he measured.


 
 Return to top of page · Post #: 6 · Written at 1:24:08 AM on 3 August 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6013

I think one of the main benefits of an earth stake these days is because in many cases the pipework (or much of it) throughout many modern buildings is plastic after the meter.

Yes, and Simpson mentions the danger inherent in old water pipes being indiscriminately replaced with PVC when the associated electrical installation is using water pipes as the solid earth.

He further mentions the effect of thyristor controllers putting DC on the mains, and thus the MEN mains earth circuit, leading to corrosion issues associated with DC and the mass of earth. (I gather that railways have a constant struggle with this electrolysis issue).


 
 Return to top of page · Post #: 7 · Written at 2:31:05 PM on 3 August 2014.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6353

Railways that feed DC to the trains would have an issue to a degree. In NSW the feed is 1,500 volts DC for heavy rail and 750 volts DC for light rail. I think it is 1,500 volts DC and 600 volts DC for the same in VIC. In QLD and WA they have changed from DC to - 25,000 volts AC for heavy rail - not sure what the light rail line on the Gold Coast is providing. SA is looking at changing to 25kV AC at some stage.

I doubt NSW will change - the system is too big for a cost benefit to be realised, especially since around 400 trains (four motor cars and four trailer cars in each train) would have to be retrofitted with new inverter equipment to convert and drop the voltage. Then there is the greater electrical losses that occur in an AC distribution system. A lot of money has also been invested in upgrading substations, rectifiers and catenaries to suit the M-Sets and A-Sets, which do consume a lot more power than the older sets.

As I write, I am reading the article in Silicon Chip. The idea that the supply authority says the installation is safe is because the fault is likely down to them in the first place. Line clamps on aerial conductors are not infallible, which is why there is a requirement that there are two line clamps on each join on the neutral conductor. If you walk outside and have a look at both your point of attachment and the aerial lines in the street generally, you will (or should) see two line clamps for each termination on the neutral but only one on any active joins.

The same should be seen when you open a junction box in the roof of your house, although in this case the twin connections will be on the earth wire instead of the neutral. All of the BP connectors in a junction box connecting earth wires should be of the two screw type. And in any switchboard made in the last 25-30 years, the earth busbar should have two screws for each termination and the neutral busbar should have two screws for the MEN link. All of this is designed to reinforce the integrity of the MEN system.

However, it only takes the corrosion of one line clamp out there in the weather to ruin things and this is the supply authority's responsibility to fix and they won't admit it lightly. The cost associated with replacing that clamp is something like $1,000.00 by the time someone there lodges a work order, issues it to a linesman, he grabs a workmate and drives out in a cherry picker, they both go up, replace the faulty clamp and then return to the office and sign off on the job. They just won't do it unless your power fails and that is the end result of privatisation and the government mandate that the supply authorities are required to provide the government with a minimum dividend each year on top of taxes.

I've honestly never seen a plumber bridge out any cuts they make although some probably so it on occasions. It's a shame there is no quick fix for this, aside from replacing all pipework with PVC but no-one would be prepared to pay for it.


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A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 8 · Written at 2:40:47 PM on 3 August 2014.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6353

I should correct Leo Simpson's interpretation of what MEN stands for too - it is Multiple Earthed Neutral, the practice of earthing the neutral conductor at more than one point to insure the integrity of this method of protection.


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A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 9 · Written at 3:57:44 PM on 3 August 2014.
Airzone's Gravatar
 Location: Maclean, NSW
 Member since 30 May 2008
 Member #: 291
 Postcount: 341

The neutral is theoretically at 0 V potential, as any grounded object, but current flows on the neutral back to the source, somewhat elevating the neutral voltage. NEV is the product of current flowing on the neutral and the finite, non-zero impedance of the neutral conductor between a given point and its source, often a distant substation.
This is typical if you live on a farm out of town, and noticable if you touch the shower rose with a cut on your hand. On the farm where my in-laws lived you would get a tingle from the washing machine, laundry tub and shower rose. Everything in the installation was fine, but that is not saying that the connections to the power poles down the line were any good also. Being near the coast in what they call "salt laden atmosphere" can be a problem with cable and connections.
I am a little disappointed in some of Leo Simpson's comments that he writes about, some obviously out of his knowledge depth before going to print.

Under AS/NZ3000, you can use single screw connectors on earths now as long as the wires fill up to 75% of the connector's hole. I always use double screw connectors, old school, and as we have been talking about, the earth is the clients protection.

A little off track:
A previous article that Leo wrote about was out of his depth and that he allowed stupid comments in the letters to the editor.
"The article on Internet Over the Power Lines", issue before last, and the stupid comments to the editor, from others that do not know, bugged me. I use Internet over the power line from my house to my shed and it is fantastic. NO you can't use it between different phases, it is not rocket science why.
My brother tried it to his workshop and nothing, we found the an old Clipsal circuit breaker was a fault, change it and works fine. Again it comes down to good connections when sending high speed date over the power lines.

Incorrect or poorly researched articles can be dangerous when we are playing with Electrickery Wink


 
 Return to top of page · Post #: 10 · Written at 4:12:08 PM on 3 August 2014.
Airzone's Gravatar
 Location: Maclean, NSW
 Member since 30 May 2008
 Member #: 291
 Postcount: 341

On the earth stakes, they have been around ever since the MEN system, they used to be lengths or gal water pipe driven down near an outside tap. They then went to copper sheathed steel stakes with a brass clamp, it then has to be painted to stop corrosion. From the stake you connect to the pipe going to the tap. Big problem now is poly pipe water installations.
Any earth bonds must be done as close as possible to the point of entry to the premises.


 
 Return to top of page · Post #: 11 · Written at 6:01:43 PM on 3 August 2014.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6353

Under AS/NZ3000, you can use single screw connectors on earths now as long as the wires fill up to 75% of the connector's hole. I always use double screw connectors, old school, and as we have been talking about, the earth is the clients protection.

I always thought that two screw connectors were always required though I always wondered about terminations at ceiling roses and GPOs - they all have only one screw and in the case of GPOs, if you twist the wires together neatly enough there is room for four wires even though I've always tried to avoid anything more than one pair in and one pair out. Nothing is worse than confronting a bird's nest of four pairs of 2.5mm2 solid core cables installed by a 'Jewish' electrician who only leaves 50mm tails coming out of the skirting board.

I've also noticed that the 'bible' requires gas pipes to be bonded now. When I did my trade that was a big no-no for the fear of an explosion if any pipework became separated, creating an arc.


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A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 12 · Written at 9:04:06 PM on 3 August 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6013

Still interested in clamp readings from members of current at the main earthing point of their systems (with some sort of load powered up indoors). Also at water meter if pipes are used for earthing.

I have two clamp meters that I picked up for a few bucks months ago in a box of assorted stuff at an HRSA auction. I need to get batteries for both of them to find out if they work.


 
 Return to top of page · Post #: 13 · Written at 9:10:34 PM on 3 August 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6013

If you walk outside and have a look at both your point of attachment and the aerial lines in the street generally, you will (or should) see two line clamps for each termination on the neutral but only one on any active joins.

No aerial lines in my street, but I note double clamps on neutrals around the area. Will look more closely at other streets as I get around.


 
 Return to top of page · Post #: 14 · Written at 10:22:58 PM on 3 August 2014.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6353

Also at water meter if pipes are used for earthing.

The pipes will be bonded to the MEN link anyway if they are metal. The earth stake is a recent additional requirement. I'll see if I can get some readings tomorrow. I missed an opportunity today with an unscheduled commitment. Perplexed


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A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 15 · Written at 10:32:04 PM on 3 August 2014.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6013

The pipes will be bonded to the MEN link anyway if they are metal.

I can't see where that occurs here with my metal pipes. I gather that the earth bonding rules for MEN installations may have changed around quite a bit since the 1980s. This place is 1982 vintage.

On the question of earth stakes, I noted at a friend's (rented) place today that under their switchboard the main earth cable is attached to what seems to me to be a rusty piece of 'reo' rod cemented in between the brick wall and the concrete path. The clamp connection is rusty, too. It strikes me as a very poor main earth. I'll post a photo it for comment.


 
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