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.
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.
Re: Ring circuits and EMF#138273 08/27/0304:49 AM08/27/0304:49 AM
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.
Re: Ring circuits and EMF#138274 08/27/0306:12 AM08/27/0306:12 AM
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.
Re: Ring circuits and EMF#138277 08/27/0311:54 AM08/27/0311:54 AM
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.
Re: Ring circuits and EMF#138278 08/27/0306:03 PM08/27/0306:03 PM
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???
Re: Ring circuits and EMF#138279 08/28/0311:32 AM08/28/0311:32 AM
Any leakage I know of returns on the grounding conductors.
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.
They can't just wander off absentmindedly???
You know, the young electrons these days...
Re: Ring circuits and EMF#138280 08/28/0312:41 PM08/28/0312:41 PM
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?