 | Home > Off Topic > another one for the sparkies... |
 
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| Caterham Member Since: 06 Nov 2008 Location: Birmingham Posts: 6264   
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just wondering what the consensus is on the following.
wiring up power and controls to indoor fan coil units for heating / cooling. basically those that are familiar with them - it's a tin box with cable knock-outs. please describe what you believe is correct method / what regulations dictate. should cables, flexes etc be poke through hole, should rubber grommet be use, stuffing gland etc. should controls and power go through same hole - bearing in mind controls cable is screened to prevent harmonic distortion etc. many thanks, |
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8th Apr 2019 7:11pm |
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| Caterham Member Since: 06 Nov 2008 Location: Birmingham Posts: 6264
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many thanks.
they've use typical 3 core flex and to my mind it needs to be a plastic stuffing gland, if for no other reason to ensure the IP2X integrity. I'd also agree a grommet would be 'can't be arsed' and personally I doubt it would fulfil the requirements of the regs.......which leads me to another subject. I need to go a 18th edition course.....but where do I find one. I'll do a search - I'm sure the local college with oblige. 
thank you.  |
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8th Apr 2019 7:54pm |
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| Rashers Member Since: 21 Jun 2015 Location: Norfolk Posts: 3341  
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And once you have done that, you can install your arc fault detection devices and surge suppression devices
It depressed me. I’m just hoping for the lottery win so I can burn my regs book. You only need to segregate mains power and control cabling if the controls cabling is not mains voltage rated. That is part of the regs and has been since at least the 15th. AC engineers tend to use that braided cable which has a two letter acronym (CC, CY?) I will look it up. The Rags does not recognise these cables so in theory, they should be highlighted as a departure on any test and inspection certificate. I don’t expect you will be doing one of those 
There’s loads of this cable out there and factories, especially food factories love the stuff. |
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8th Apr 2019 8:26pm |
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| Caterham Member Since: 06 Nov 2008 Location: Birmingham Posts: 6264
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got another one if you don't mind.
this is a domestic situation although I don't suppose that makes any difference. new build house. buried duct installed for elec supply to gate. brother in law quoted silly money to install cable. they've suggested 300 mtrs of 50mm swa. my reckoning suggests 10mm will kill it. but the question is does it even need to be swa as it's in a duct. ie would double insulated not be sufficient? I know generally everyone simply chucks in swa but that doesn't necessarily mean it has to be - does it? many thanks. |
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28th Apr 2019 10:23am |
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| leeds Member Since: 28 Dec 2009 Location: West Yorkshire Posts: 8578
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What are you powering on the end of a 300 metre 50mm2 SWA cable, a bloody battleship?
OK you can expect some voltage drop. 300m run in ducting/underground of 50mm2 cable is good for 30 amps according to Doncaster cables What current do you actually require? Would it be easier to have a solar panel, storage battery and a 12 volt motor correctly geared? Or an armed guard at the end of your 300 metre drive? Brendan |
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28th Apr 2019 12:18pm |
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| Caterham Member Since: 06 Nov 2008 Location: Birmingham Posts: 6264
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thanks for the response Leeds. that's exactly what I thought. the bro-in-law is looking at some electric gates (well one actually) and perhaps some lighting so as you've already concluded someone has surely well and truly over estimated the load and volt drop (or they're simply looking to make a bigger job out of it than is necessary - and therefore presumably more profit. in terms of volt drop you might get away with something as small as 2.5mm as I don't anticipate the actual load to be much more than 1,000 Watts. My biggest concern (and I've forgotten how to work it out) is disconnection times which relate to earth loop impedance ie size of the cpc (circuit protective conductor). looking at costs based on the smaller cables I'm inclined to go with the SWA cables as the savings on non armoured aren't so massive and the SWA is better suited plus helps with the earth impedance. My gut feeling says 10mm to give some scope for a higher load and will better meet the requirements for the CPC impedance but I don't know what those requirements are |
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28th Apr 2019 12:38pm |
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| Rashers Member Since: 21 Jun 2015 Location: Norfolk Posts: 3341
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The disconnection time will be determined by the earth fault loop impedance at the source of the supply. You also need to be mindful of exporting the earth outside of the equipotential bonded zone if it is a TNC-S / PME supply. You can extend the equipotential bonded zone but this would require using a cable with an integral earth wire (3 core SWA) the same size or greater then your main bonding conductor. Domestically this will probably be 16mm2. Might be easier to supply the gates and lights in 2 Core SWA, supply with a 30mA RCD, isolate the armouring at the end of the cable and install an earth electrode. Turn it into a TT earthing system.
Hope that helps. 50mm2 does seem a bit excessive but 300m is a long old run. |
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28th Apr 2019 2:51pm |
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| Caterham Member Since: 06 Nov 2008 Location: Birmingham Posts: 6264
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many thanks Rashers.
isn/t the disconnection time determined from a combination of the max fault current and the earth loop impedance which is a combination of the proposed circuit AND that associated with the supply known as external earth loop impedance? as you say a local earth arrangement might be far more suitable - thank you
....and it might not be of course depending on how easily the disconnection time can be achieved - perhaps?  |
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28th Apr 2019 3:19pm |
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| Rashers Member Since: 21 Jun 2015 Location: Norfolk Posts: 3341
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Haven’t got my regs book at home (no trouble sleeping so I leave it at work
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Except for a few types of circuits, disconnection time is 0.4 second. If your maximum earth fault loop impedance is below those prescribed in chapter 41 of the IEE Regs for the 1) Type of circuit and 2) The type of protective device, you have complied with disconnection. I would always supply something like you describe from a 30mA RCD. |
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28th Apr 2019 4:38pm |
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| Rashers Member Since: 21 Jun 2015 Location: Norfolk Posts: 3341
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Just re-read your post Caterham. Maximum fault current or prospective short circuit current is inversely proportional to the earth fault loop impedance. Ohms law. With a constant voltage (230 Volts), the higher the earth fault loop impedance, the lower the prospective short circuit current and vice versa.
Prospective short circuit current, from a design point of view only really affects the breaking capacity of the protective device (how much fault current the MCB/RCBO or fuse can take without self destructing). Most MCBs and RCBO’s produced these days will easily withstand a prospective short circuit current which would be recorded in most domestic properties. High values which become worrying are only found when you are smack bang next to the HV transformer. Saying all of the above, the further you get from the transformer, the higher the earth fault loop impedance. Hope that makes sense |
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28th Apr 2019 4:55pm |
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| Caterham Member Since: 06 Nov 2008 Location: Birmingham Posts: 6264
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makes perfect sense......I just need to go figure how to put it into practice so to speak. and like yourself I would also opt for the rcd /rcbo - many thanks. |
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28th Apr 2019 7:21pm |
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| Caterham Member Since: 06 Nov 2008 Location: Birmingham Posts: 6264
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just a thought. you mention 0.4 disconnection.
did it not used to be; 0.4 for socket outlets, 5 sec for fixed circuits? |
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28th Apr 2019 7:22pm |
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| Rashers Member Since: 21 Jun 2015 Location: Norfolk Posts: 3341
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Yes, you are right. It used to be when it was called EEBAD (earthed equipotential bonding and automatic disconnection - I think I got that right?)
I believe it was the 17th Edition when all circuits in a TN earthing system under 32 Amps were made 0.4 sec (0.2 sec for TT earthing systems if you live in the country). 5 sec is for circuits above 32 Amps. A lot has changed in the last couple of editions. Sadly, I believe the quality of electrical installations is not reflected in the efforts the IET have put into these changes. |
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29th Apr 2019 3:06pm |
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| Caterham Member Since: 06 Nov 2008 Location: Birmingham Posts: 6264
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so I'm a bit out of date then. I'm trying to get on an 18th edition course but struggling to find one local at the moment. |
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29th Apr 2019 3:34pm |
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