Greetings. This is my first post here. My area of professional concern is EMI from power frequency magnetic fields generated by certain wiring errors found frequently in the U.S.A.

I read with interest the excellent threads (from 2001)on the Ring mains circuits used in the UK. I understand and would have predicted the disadvantages listed. I also understand how they came into being in the '40s.

I noted that it was mentioned that a stronger magnetic field would be generated by the unequal phase and neutral currents which would be created when there was a break in one of the conductors. This was given little weight, though in the USA such fields do interfere with computer monitors and other sensitive electronic instruments.

A reasonable assessment of the health concern is not available to most electricians since the controversy has caused the media to under-report the ongoing research. Opinions are generally formed due to attitude rather than knowledge of the research.

There was one assessment of risk which is worth knowing about: in 2001 the International Agency for Cancer Research (IARC), an agency of WHO (World Health Organization) reviewed the recent mega-studies of the many epidemiological studies conducted over the years, and by combining them came up with a solid connection between childhood leukemia and magnetic field exposure of 0.4 microTesla (4 milligauss - mG). Thus this conservative body designated this level as a possible carcinogen.

In the USA our Code almost completely excludes "net current" circuits and their magnetic fields, so here is a difference, since the UK Ring circuits would not be allowed, since the conductors are paralleled (NEC 310-4).

So now I will see if there is any interest in this post.

Karl Riley

Hello Karl, and welcome to ECN.

I have to confess that I'm not particularly well-read when it comes to the issues of EM fields on health, although naturally I've followed some of the stories over the controversy. Like many, I suppose I've seen the arguments from both sides and decided to take an agnostic view of the situation.

With regard to EMI to electronic equipment, this is something which has concerned me, as I frequently work with sensitive equipment involving very low signal levels. The problem of induced 50/60Hz hum is one which often takes quite a lot careful work to sort out.

The British ring circuit can certainly result in quite strong EM fields if one conductor is broken, but there can also be imbalance due to less-than-ideal connections. It takes only a little more resistance on one terminal to result in the split of current around the ring being different on the line and neutral.

Karl,
Your point is interesting. In the UK Section 607 of BS7671 2001 deals with high protective conductor currents, (often associated with IT equipment such as computers). Where a number of computers are to be connected to a circuit then the cumulative "leakage" current may exceed stipulated upper limits. In this case, in order to protect against possible shock hazards, high integrity earths are required.
There are a number of options but what is being favoured by many specifiers is the adoption of the ring final circuit principle, simply in order to achieve dual return earth paths.
I note your comments with respect to EMI which I ran past a colleague. He acknowledges that building earths should not be run as a ring in general but that the restricted length of a ring final would render it of minimal consequence.
I would appreciate yuor comments.

Interesting. I wonder if this explains why the Swedish wiring reqs require that electrically parallell conductors run parallell physically too.

Hello Karl, and welcome. I was wondering how long before you would post this here. For the record...I stand by my first thoughts that they are a parrellel circuit and therefore illegal in the U.S.

[This message has been edited by Ryan_J (edited 08-27-2003).]

Thank you all for the replies. Pauluk, yes, I can see that any impedance differences would send more current one way and therefore cause some net current and its magnetic field.

Ryan, I agree.

B-H yes, if the paralleled conductors were run in the same cable there would be less net current. However, as Pauluk mentions, if impedance of connections vary, even then net current would be set up.

(Oops, forgot the name of the other fellow), but I would first like to know what "leakage currents" are. ?? Most computer monitors start to jitter at anywhere from 0.5 uT to 1.2 uT. This would be caused by a net current in a circuit of 0.25A to 0.6A just one meter from the computer.

Karl

Leakage current is the current that flows out in the circuit and doesn't come back. This comes from poor insulation between earth and live conductors. There will of course always be some leakage current, since no material is a perfect insulator.

It's typically a concern when it comes to wiring protected by an RCD (GFCI), since it is designed to trip if the incoming and outgoing currents doesn't add up. A PC will leak about a 1mA and the recommended maximum level on a European RCD is 10mA, as it's designed to trip between 15 and 30mA. This means that you can only have about 10 computers on one RCD.

C-H, I had never heard of current leaking out and never returning, except in RF propogation. Any leakage I know of returns on the grounding conductors. The voltage differential pretty much forces currents to return to the transformer, no? They can't just wander off absentmindedly???

Karl

Most of it, yes. But the leakage can go to any earthed object, such as a water pipe, since the transformer is earthed. The earth wire is not an live wire and does not pass through the RCD. If all electrons were well behaved enough to stay in the wires, there would never be any risk of electrocution.


You know, the young electrons these days...

Are you sure C-H? 1mA seems very much for a PC.
I have also heard that HF ballasts have a high leakage currents. The reason being because of the electronics inside. This might also explain the reason for the high leakage currents in PCs?

The leakage current would mostly come from filters in the power supplies. The same should be true of HF ballast.

I'm not entirely sure about the 1mA. When Googled for leakage currents I got two results: A French site that claims it's about 2mA per computer and an
article on ePanorama which claims that it's typically less than 1mA.

The leakage current can vary considerably with the design of the filters.

It doesn't take very much capacitance line-to-ground to get a current of 1mA flowing:

About 0.014uF for a computer running on 230V at 50Hz, and around 0.022uF for a 120V 60Hz unit.

Is there any evidence that leakage currents dissappear and dissipate into the earth? Do they not make their way back to the transformer?

Karl

Yes, the leakage current would have to go back to the transformer. Thinking about our other current thread on different grounding methods, the leakage from the filter capacitors will get back to the transformer neutral:

1. In a TN-C-S (PME or American system) by way of the building EGCs, N-G bond, and supply neutral, along with any parallel paths created by pipework etc.; -or-

2. In a TN-S system, by way of the EGCs and the independent ground right back to the xfmr star-point; -or-

3. In a TT system through house EGCs and the actual earth itself by way of the local ground rod and the other ground rods on the network's neutral.

There's another quite strong source of EM fields in many British houses that we haven't touched upon yet: The lighting circuits.

These days, all cabling is (or should be) run so that current flows out and returns on adjacent conductors in a cable, thereby canceling the field as much as possible.

In the past, however, it wasn't unusual for lighting circuits to be wired with single-core cables where the phase and neutral could take quite different and widely separated routes around the house (I believe that some early knob-&-tube systems in the U.S. were wired similarly).

There are a large number of houses in my imediate neighborhood which were wired (or rewired) in the 1960s and use this method.

Although we have lighting circuits separate from those feeding general receptacles, there can still be a few amps flowing at some points -- Enough to generate a sizable EM field in the vicinity.

Yes, the knob and tube wiring I have seen is as you describe, with large loops of fields from small amperage. Though this method was abandoned in the 40s, there are a lot of buildings in some areas such as San Francisco which like to preserve older buildings where one finds this wiring still. It is legal to leave it or even add on to it, but not to install new.

It is interesting to consider that early house residents may have been exposed to higher EM fields even with minimum electrical usage than those in modern high-load houses which are wired according to present Code. An interesting study did show that cancer has risen following early electrification in areas previously not served. With knob and tube, the early users were getting a good dose of magnetic fields!

Karl

That may well be true.

To throw a monkey-wrench into the argument, however, I would add that in the radio/electronics field it seems as though some modern manufacturers could take a lesson from the past. Techniques which were once commonplace seem to have been forgotten.

Look in some equipment now and you'll find AC wiring from power transformers (both primary and secondary) running on separate conductors not particularly grouped.

In the old days, the effects of EM fields from such wiring were widely understood. Look at the way AC power was wired to the filaments in old tube equipment, for example, and you'll see a tightly twisted pair run close to the chassis (they didn't use the chassis as a return path, even though one side of the filament wiring was grounded to it).

Keeping the EM field from the filament wiring as low as possible was important when very low-level signals are being carried on adjacent wiring.

That's curious, since manufacturers have to meet standards for EMI emissions and protection.

Of course the effect on living space is small.

An electrician I know used to twist NM (Romex) conductors in house wiring to reduce magnetic fields, thus costing his customers much more in his labor. He did not realize that twisting has no effect on net currents, and is not needed when the wiring is correct.

Karl

Hey Welcome Karl!,
What an interesting topic you bring up.
EMF's have always been a thing that I have been worried about, with respect to the HV work that I do, however, I am always a fair distance away, using a Hotstick, but it kind of makes you wonder though, just how harmful are these things?.

Thanks for the welcome. I too wonder why I am standing in fields of 100+ mG clamping my ammeter around service conductors. I guess it is like a fireman: the fire is dangerous, but it is your job to go in.

About the actual dangers, it seems to affect young children most easily (leukemia). Statistics for adults (leukemia, brain tumours, Alzheimers, etc) are much weaker and have not convinced the community in general. Also, I would think it depends on whether one's family has a history of cancer. Mine have usually died of heart (and smoking).

Believe it or not, there have been literally thousands of studies on the biological effects of power frequency magnetic fields (and some on electric fields) in the last 15 years, and I mean thousands. So one has to read a journal such as microwavenews.com to keep up with it. And politics is highly involved in funding and also reporting of results.

I don't get into discussions about the health angle much any more since I discovered early on that most high fields in buildings are caused by Code violations. This lets me trace the error and get it fixed. Case closed.

Of course in the UK I would be a bit upset by Ring circuits and the separating of hot and neutral conductors in lighting circuits. Both woulod be Code violations here if installed today.

Karl

I recall one of those "consumer" TV shows a year or two ago which looked into the subject of EM fields and health.

It wasn't a serious scientific discussion, but one thing which has stuck in my mind is the home where they found a very high gauss level around the head of a bed in one of the children's rooms. The service entrance and meter were located on the other side of the partition.

I've been surrounded by electromagnetic fields of various types in my work for over 20 years, and before that through many childhood years of "playing" with radio and electronics. I guess I've never really stopped to think about possible side-effects.