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Post Time: 16/06/2008 20:40:01 |
KeiraN
Total Forum Posts: 16 |
I'm interested in fitting a Demand switch to one or two of our ring main circuits to eliminate EM fields overnight. I've checked the circuits to make sure there are no devices which use current in standby mode etc. However, I understand that ring mains are rated at 32A, and the Demand switch available from the On-line Powerwatch shop are only rated at 16A. These would therefore seem to be the incorrect specification.
Is this assertion correct and if so, can you recommend suitable Demand switches for 32A ring main circuits.
Thanks
Tony |
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Post Time: 27/06/2008 10:06:47 |
alasdairP
Total Forum Posts: 173 |
I do not know of a 32A Demand Switch. We (nor the IET (!), though they do allow them still to be used) do not recommend the use of ring final circuits as they can produce elevated magnetic fields. SAGE recommended that they are not used any more and should be changed to one or two 16A radial circuits - their reasons can be found in the SAGE Appendices and in our House Wiring for Low EMFs booklet that can be freely downloaded from this website by Powerwatch Forum members. The easiest way is to split the ring at about its mid-position and then feed the two halves as two 16A radial circuits. This will require another MCB in the consumer box, but you will need extra ways to accommodate the Demand Switch(es) so that should not be a problem.
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Post Time: 02/11/2008 13:19:19 |
KeiraN
Total Forum Posts: 16 |
Thank you for your previous answer, sorry to be slow in replying.
I downloaded 'House Electrical Installations and EMFs' is that the booklet you meant, as I couldn't find 'House Wiring for Low EMF's booklet'. If it is the right article would you mind telling which page the SAGE comments about 16A v 32A is?
Thanks very much
Keira |
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Post Time: 02/11/2008 13:49:36 |
KeiraN
Total Forum Posts: 16 |
Hi again,
Forgot to ask you to explain a bit more about how a 32A demand switch would "produce elevated magnetic fields"? Or is it just the "Ring Final Circuit" that does that? Our problem is that if we were to use 2 x 16A damand switches and re-wire our one ring main (circuit) into two radial circuits we would would have to pull up carpets/floorboards in a number of rooms.
Is there any way round this? Many thanks for your advice.
Keira |
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Post Time: 04/11/2008 20:56:36 |
alasdairP
Total Forum Posts: 173 |
Apologies - the newer (and current) title is House Electrical Wiring and EMFs as you discovered. My memory plays up re. titles at times. Page 17 states the SAGE view to: "Wire power circuits as radial circuits instead of ring circuits". Our article needs slightly updating again (when doesn't it?) to put SAGE into more context. If using demand switches, then you should use the diagram on page 9 showing a ring circuit split into two radials - just use the 16A demand switches instead of the 20A MCBs. 2 x 16 Amps is more than enough for most modern homes.
www.equilibra.uk.com/demand.shtml sell a 20A nominal demand switch from Biologa if you need the extra current.
Please note that both our (Gigahertz) and the Biologa Demand Switches need to be installed on a circuit that is also protect by a fuse/MCB overcurrent protection device of 16A or 20A respectively. The Demand Switches do NOT act as an over-current fuse.
You don't need to take up floorboards - just work out roughly where half the ring "is" and disconnect both ends of the cable section there that makes it into a ring - best done on a link between two rooms. Then you have two separate radial circuits that you can protect using 2 x 16 Amp demand switches in the consumer (fuse) box. No floor boards need to be lifted - just two sockets unscrewed and one section of wire disconnected at both ends (it can be left in place after disconnection).
I hope this helps.
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Post Time: 04/11/2008 21:41:51 |
alasdairP
Total Forum Posts: 173 |
I forgot to say that the "elevated magnetic fields" are produced by the ring circuit, not the demand switch. The trouble with ring circuits is that, however well they are wired, very small differences in impedance (mainly resistance) around the Neutral and Line rings unbalances the "go" and "return" currents and this increases the magnetic fields for any given load. This is clearly shown in the graph on page 5 of the article.
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Post Time: 05/11/2008 11:36:44 |
KeiraN
Total Forum Posts: 16 |
Thank you very much, that has clarified things. Will be back with more Q's later no doubt.
Keira |
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Post Time: 06/11/2008 12:02:26 |
KeiraN
Total Forum Posts: 16 |
Hi,
If you do what you suggest to the 'Ring circuit' and disconnect it making it into 2 radials, how does the 'Return' bit work. Wouldn't you have to add more wiring for the return? Also, if we have used several junction boxes in the ring circuit will that be a problem? Hope this makes sense, sorry.
Also, I hope you don't mind but could you clarify whether 'CY1.50mm2 3 core, Flexible PVC Insulated and Sheathed Power Cable' is Armoured or just screened?
I understand also that 'Armoured' cable is better at shielding Electric fields (not magnetic) than the more flexible 'Screened' cable , is that correct?
Thanks very much again
Keira |
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Post Time: 12/11/2008 20:37:27 |
alasdairP
Total Forum Posts: 173 |
Sorry about the delay. A ring circuit goes in a complete ring - starting at the consumer (fuse) box and going around all the sockets and then return back to the same point in the consumer box so that the "ring" is connected and hence "fed and returned" at both ends. "Cutting" it in the middle just means the each half is fed separately from the consumer box. The electricity flows (goes) out on the "line/live/phase" wire (red or brown) and returns back on the Neutral (black or blue) in the same cable.
Junction boxes are not a problem. They either just join parts of the circuit or add short radial "spurs" to feed one or two sockets not fully on the actual "ring" loop. These will work as normal after the "ring" is broken.
The correct cable to use in the UK (I am not sure about other countries regulations) is now BS8436. This is just screened (not armoured) but works just fine. Armoured cable is VERY difficult to work with and could not easily (if at all) be connected safely into normal switches and sockets. If you want the highest level of screening and protection you would need to wire in MICC (mineral insulated) cable where the Phase and Neutral are contained inside a solid copper sheath. There are special glands that have to be used with this when you connect it to switches, sockets, etc. It is easy enough to obtain, but is very expensive to buy and to have installed. It is used for safety critical applications including fireproof wiring.
The main manufacturer of BS8436 cable is Pyrelli/Prysmian with LSX and Earthshield cables.
Doncaster Cables are another manufacturer that makes BS8436. There is another firm in Hertfordshire, I believe, that manufacture cable to BS8436. I do not have their details to hand.
Also:
ALSECURE® BS8436 NXS 2,3,4 core, 1.5-6mm
http://www.nexans.com/eservice/Navigate.nx?navigationId=120396&CZ=UK#top
Also:
http://tpwcc.com/flexishield.html
I think it was about £105 per 100 metres for 2.5mm2 T&E last time I heard a price.
Best wishes
Alasdair
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Post Time: 12/11/2008 22:27:01 |
KeiraN
Total Forum Posts: 16 |
Thank you very much again for your time and information. I'm another step along in my understanding. Best regards Keira |
