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Old 16th Jan 2020, 4:16 pm   #101
HamishBoxer
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Default Re: Sheds, damp and test equipment

I have used foil backed plaster board though it certainly was not earthed. No problem if you are using an external aerial.
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Old 16th Jan 2020, 4:28 pm   #102
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Default Re: Sheds, damp and test equipment

Quote:
Originally Posted by dominicbeesley View Post
While we're on the subject of "sheds" and safety earthing I asked a question recently but got no replies - while the earthing gurus are here might I take the opportunity to ask again.

I have a room above a detached double garage that I'm making warm and dry (it's already dry but not warm) for my collection and office. I am insulating it with foil backed insulation (outer layers of thick aluminium containing many layers of blown plastics and metallised film. This will be covered over with plasterboard but does it need to be earth bonded - if so what gauge of wire and how to attach it? Should the bond go to the main fuse box or can it go to a ring main earth - which is what will happen by default where it contacts conduit and back boxes.

Also there are two large exposed steel girders in the roof which are easily touched (I bump my head regularl). These appear not to be bonded - should they be? Again ideas on bond wire gauge.

As far as I can tell the earth for the garage comes via the sheath of an armoured cable from the house ca 10m away

Thanks

D
The Earth wire gauge use to be 6mm but 10mm is more likely to be used these days in many bonding applications.

Although not sure if the girders are best left floating if you touch them with your head while accidentally holding a live wire LOL..hopefully the RCD would react quick enough.

Don't think the insulation needs to be bonded if you are covering it up with board, but you can get epoxy conductive glues if you so wish and if you are carrying cables through it then thread it through conductive earthed trunking.

That is just an opinion... the previous edition of the IEEC wiring regulations is here free

https://elektroinstalacije.files.wor...egulations.pdf

but you might need a holiday after reading that
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Old 16th Jan 2020, 4:39 pm   #103
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Default Re: Sheds, damp and test equipment

No expert, but I can't see it's practical to earth insulation board foil, or required. I know a few builders who advise me on things and I've insulated a few false ceilings I've built, none has ever mentioned earth bonding the insulation boards.

Depending on the structure, imagine how many separate pieces of Celotex there might be, and lots of it has foil on both sides. If not doing it has made me non compliant then so be it
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Old 16th Jan 2020, 4:46 pm   #104
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Default Re: Sheds, damp and test equipment

I was in the building trade for a number of years as a chippy, installed loads of foil backed insulation and foil backed plasterboard, never seen any earth bonding for either, the foil on plasterboard is I think plastic.

Extraneous conductor....bond to the MET so far as I know.

I exported mains from the house to a large shed in the garden, house supply is TN-C-S, no extraneous conductors in the shed, sometime later I had an LPG gas hob installed, the supply pipe for that was copper so classed as extraneous (metallic contact with LPG bottle, LPG bottle on the ground outside) the scenario of a supply neutral interrupt on the DNO's line crossed my mind so I TT'd the shed supply.

Lawrence.
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Old 16th Jan 2020, 5:03 pm   #105
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Default Re: Sheds, damp and test equipment

A mate of mine came off the company PME earth system after the neutral went open circuit down the street and apparently because of a slight imbalance in the phases, he had over 400 volts on all his appliances, with some disastrous results - he said never again! He now has his own private earth stake driven deep into the ground and no more problems.
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Old 16th Jan 2020, 5:32 pm   #106
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Default Re: Sheds, damp and test equipment

Dominic,
"Exposed Metalwork" bonding means just that. Foiled backed insulation, covered with plasterboard doesn't need bonding. I assume(& hope) that your detached double garage is supplied with a dedicated sub-main from the house's meter/distribution board. These days, after the Utility Company's meter there usually is an 80 or100A D/P isolation switch followed by two connector blocks(we call then Isco blocks) which can take several D/I single core cables up to 25mm cross section. (we call them "Tails"). These tails can go to your main house Consumer Unit, and to a Sw/Fuse Unit controlling the sub-main out to your garage. The Utility's Cut-Out(Sealed Fuse Unit) will also have a big brass Earth Block on the side. The Min size of earthwire from your CU would be 16mm or 25mm. The min size of earthwire from the sub-main should be not less than the size of the L & N conductors. Big double garage - probably 10mm. Then inside your garage - those big steel beams - must be bonded with 10mm earthwire at the nearest possible point. Likewise - any incoming gas or water pipes -10mm. You might say - hey, they're blue alkathene, but they'll change to copper or brass somewhere, so that's the bonding point.
This is just a rough guesstimate/description. The Regs have accurate tables which relate VD/m to the expected current use & hence the size of cabling needed. Plus min sizes of earth bonding.
Fixed Metal workbenches, big air compressors, welders, motorised doors, powered car lifts, etc. all come under the requirements for main earth bonds back to the garage's distribution earth block(either the big brass bus-bar inside the CU, or a dedicated Isco Block) - not to a wee nearby double socket's earth terminal. The power to big machinery might(should be) be via fixed dedicated safety switchgear.
If you are in any doubt about garage safety - contact your nearest NICEIC & Inspection Registered Electrician ASAP.

Regards, David
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Old 17th Jan 2020, 2:24 pm   #107
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Default Re: Sheds, damp and test equipment

Thanks all,

I think I'll have to crack open the distribution boards to see how it is wired the previous owner thinks the earth is via the shield of the armoured cable and the house is PME according to the label on the mains.

The supply to the garage is from a breaker in the house distribution board (useful when I forget to turn the lights off and it is p***ing down outside). At some point I might get someone in to turn the garage into a TT installation with an earth spike - it is surrounded by nice _very_ wet ground. I'm guessing TT would be safer when I start setting up aerials and RF earths?

I will run a 10mm earth bond to the girders - even though I will probably end up boxing them in at some point.

I will be getting someone in to do an electrical inspection at some point soon as I'm really not happy with the inspection that seller of the house paid for - there are a lot of faults RCD trips if some of the outside circuits are enabled (I'm guessing damp ingress in the outside switches or lumieres) and one circuit trips the breaker. I would have thought either case to be an automatic fail.

Again, thanks for the advice

D
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Old 17th Jan 2020, 7:27 pm   #108
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Default Re: Sheds, damp and test equipment

Dominic,
You might find that your Electricity Company requires you to maintain the PME right through to the Garage, thus complying with the Electricity Supply Act. Likewise, your house insurance company might not be too happy if you were to change the garage supply to TT, whilst the house remains as PME.
In years past, it was common acceptable practice to rely solely on 2 core SWA cable's wire sheath as the earth conductor, using brass compression fitting with glands & the "knock-out" holes in metal switchgear. Alternatively, with moulded plastic consumer units, "figure of eight" flanges were fitted at the ends - facilitating fitment of earthwire tails. Nowadays, conscientious electricians fit 3 core SWA cable, with the spare conductor as the "continuous main earth". Belt & Braces.
There is heaps of RSGB info on earthing & dedicated mains supplies for amateur radio equipment. Likewise, this Forum's sub-section on amateur radio, or VMARS, might reveal a wealth of sound advice.

Regards, David
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Old 17th Jan 2020, 10:02 pm   #109
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Default Re: Sheds, damp and test equipment

PME here. I run QRP off a 12v 7Ah SLA pack and charge it with a floating bench supply. Avoids having to deal with any issues. You can obviously scale this solution up if need be.

There’s 100V to true earth. Moving house very soon thank goodness.
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Old 18th Jan 2020, 1:42 am   #110
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Default Re: Sheds, damp and test equipment

Post #105 gives an example of the voltages your house mains earth can get to with a fault in the PME supply to the house. RCDs or MCBs will not trip, isolation transformers or power supplies with floating outputs will not save you because the problem is in the earth connection, not the power connection. The fault may well be completely beyond your property.

Your only protection for these earth faults is with equipotential bonding, ie connecting together with stout wires everything in your house which might have a connection to ground. With everything at the same potential, you won’t get a shock. Everything includes RF earths.

If you separate the RF earth from mains earth, then with an earth fault in the supply to your house you could have your radio equipment sitting at ground potential, and everything else in the house sitting at a substantial voltage above ground, an obvious shock hazard. It makes no difference how, or even if, you get power to the radio equipment.

Sorry to be repeating stuff from earlier, but it’s not always obvious, especially to newcomers, that using RCDs, isolation transformers, or even battery power doesn’t always make everything safe. You still need to bond all your earths, however you power your radios.

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Old 18th Jan 2020, 10:48 am   #111
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Default Re: Sheds, damp and test equipment

Mr Bungle, amateur radio-wise, I just work an 80m AM rig(Coastal Radio Nimbus Tx, and an Eddystone 730/4 Rx) feeding an 80m Doublet. My only interest is in old ex fishing boats HF AM equipment. So my workshop, as well as a normal 240V mains supply, is also wired for a totally independent 24V DC supply. Outside, under the grass, I have a matrix of bare copper 16mm cable line-tapped to copper earth rods. This is soley for the Tx & is independent of the PME Earthing. However, extensive tests reveal that the separate earths are only a max of a couple of ohms apart under dry conditions, and less than 1V ever floats. When damp or wet - zero.
100V to earth, eh ? Jesus, I'd be waring two pairs of underpants plus thick welly boots !

Regards, David
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Old 18th Jan 2020, 5:15 pm   #112
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Default Re: Sheds, damp and test equipment

My theory.
One of the worst things to cause problems with damp and condensation is changes in temperature. You go in your shed, turn on the heater stay in there for a few hours. The air you breath out is very high in water vapour and the heater has made it possible for the air to hold a lot of moisture.
When you leave the warm air cools and the moisture has to condense somewhere. Most of the equipment in the shed will not have warmed up much during the time you are in there working so is still cool. That is where the moisture will want to condense on the metal chassis of the equipment.
You cant beat good ventilation to help stop moist air from condensing on surfaces in a shed.
Of course the shed has to be otherwise watertight.

Mike
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Old 19th Jan 2020, 6:37 pm   #113
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Default Re: Sheds, damp and test equipment

Quote:
Originally Posted by Techman View Post
A mate of mine came off the company PME earth system after the neutral went open circuit down the street and apparently because of a slight imbalance in the phases, he had over 400 volts on all his appliances, with some disastrous results - he said never again! He now has his own private earth stake driven deep into the ground and no more problems.
The electrical regulations in the US require two eight foot earth ground rod s to be driven six feet apart and bonded to the neutral bar of the consumer unit.
There is a lot of regulations regarding earth connections or grounding.
I'm trying to remember all the requirements of grounding, as it changes every three or so years.
Dave US radcoll1 and retired electrician.
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Old 20th Jan 2020, 12:00 pm   #114
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Default Re: Sheds, damp and test equipment

Thanks all for the further replies.

Quote:
Originally Posted by David Simpson View Post
Dominic,
You might find that your Electricity Company requires you to maintain the PME right through to the Garage, thus complying with the Electricity Supply Act. Likewise, your house insurance company might not be too happy if you were to change the garage supply to TT, whilst the house remains as PME.
Is that true even if it is detached, there's a good 10 to 15m between the garage and the house.

Quote:
Originally Posted by David Simpson View Post
In years past, it was common acceptable practice to rely solely on 2 core SWA cable's wire sheath as the earth conductor
I need to check this but it does look that way. If it is should I just pull another separate earth line through the conduit from the house?

Mr Bungle, at present I'm SWL only but that is mostly on valved equipment.

Re PME earth faults - how often does that occur. I can see that David Simpson's assertion about a few ohms beween RF and PME earth would not help in a fault!

Also, equipotential bonding - would that help in the ground floor where I would be standing on a dampish concrete floor? Also, what about power tools in the garden?
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Old 20th Jan 2020, 1:22 pm   #115
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Default Re: Sheds, damp and test equipment

I have 3 outbuildings all of wooden construction to house my large collection of mainly wooden pre-war radios. keeping them dry is of the upmost importance.
The outbuildings are all on blocks that suspend the structures above the ground so air can circulate. The underside of the flooring is lined with roofing felt just to be on the safe side. All the walls are insulate with polystyrene and plastic sheeting as a barrier to stop any damp getting in. I have low wattage heaters permanently on during the winter months. This effort has paid off, my oldest construction is now twenty years old and nothing inside has deteriorate in the slightest.
The added benefit of having such well insulated "sheds" is these are nice places to mess about with my restorations on cold damp winter days.
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Old 20th Jan 2020, 4:50 pm   #116
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Default Re: Sheds, damp and test equipment

How often do PME faults occur - well there are 2 instances mentioned on here very recently, so it can hardly be called unheard of.

When they do occur, the problem is that none of the safety devices will trip, and with a substantial voltage on the mains earth, it could look like you've had a power cut. Since the thing connected to true ground doesn't need to be connected to power, then isolation transformers and battery power don't help. Your safety depends on the earth bonding. A bit of 10mm2 cable to perhaps save a life ?

My car has airbags which have never been used, but I'm glad they're there.

I have wondered about damp floors - maybe you rely on insulation from your shoes, but with 240V on your protective earth, but you could still get a nasty shock. Not really a good environment for working on electrical equipment. For power tools, most lawnmowers and suchlike seem to be double insulated with no connection to the mains earth.

Stuart
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Old 21st Jan 2020, 12:59 pm   #117
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Default Re: Sheds, damp and test equipment

I actually open my shed up once a day to let the air circulate in winter ,it stops that mouldy dust that sometimes gathers if its not been used for a while .
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