Electrical Earth in Observatory

[kirkster501]

Hi, when running armoured cable from the house consumer unit (via appropriate Armoured cable box and gland) should the earth be at the house end or the Obs end?  Should the Obs be using the Earth of the house or should  an Earth be provided at the Obs as well and then Earth both ends of the Shield of the Armoured cable?

Thanks, Steve

[Zakalwe]

As with anything electrical, my advice is always "If you have to ask that type of question, then you need a sparky to do the work".

I think that you will also need Part P certification to install a power installation in a garden.

http://electrical.theiet.org/building-regulations/part-p/faqs.cfm

<edit> A power supply in a garden IS a notifiable under Part P regulations. So it has to be installed by a "competent" person and checked.

http://www.diydoctor.org.uk/projects/partp.htm

[IckleImp]

Hi

The armoured cable should be connected the the earth in the consumer unit, this will provide an earth in the obsy if it is correctly installed

[kirkster501]

The obs will be connected via 20m of 2.5mm SWA.  I need to do more reading up on this.

[Owmuchonomy]

Just to top up Zakalwe's point check your local authority situation. Part P regulation has been abandoned by many LAs because it is unenforceable.

[JamesF]

Just to top up Zakalwe's point check your local authority situation. Part P regulation has been abandoned by many LAs because it is unenforceable.

I'd not heard that though I can certainly believe it.

James

[kirkster501]

The Obs will effectively at the end of the SWA as one long extension cable .  So I was thinking a metal box at each end terminating the metallic gland of the SWA.  Then at the house end it would connect via a flying plug that would connect into an RCD protected 13A plug.  At the Obs end the 2 way consumer unit feeding the obs ring ccts will connect to the metal box terminating the SWA.

I WAS an electrician many, many moons ago - gave it up after my apprenticeship!  I have the wiring skills just not the familiarity with the current best practise.  Need to read up on it. My main area of concern is if the Obs need its own earth or it can use the earth of the house along the SWA Earth wire and sheath. Should the obs be earthed at the Obs end by using a earth rod there is what i need to determine.

[gnomus]

The obs will be connected via 20m of 2.5mm SWA.  I need to do more reading up on this.

Steve

I know very little about this, and I am, therefore, getting an electrician to fit my cabling - indeed he even turned up to bury it for me.  I remember having a conversation with him, and he thought that 6mm armoured cable would probably be sufficient for the job.  In the end we went with 10mm, just to be on the safe side (and it only added a few tens of pounds to the 35 metre or so run).  

By the time you get your various bits and pieces in there (and I know that you are saving up for that 10 Micron mount) you may find that your power requirements are a little more than you might think.  I'm trying to future proof as far as possible.   

[mbhyde]

As a electtician i would earth the SWA at the consumer unit but insulate it

at the obs end and put a earth rod down at the obs end with a RCD at the obs end

[Paul M]

As a electtician i would earth the SWA at the consumer unit but insulate it

at the obs end and put a earth rod down at the obs end with a RCD at the obs end

As an electrician in what now seem permanent exile, I agree with this.

It is true that most LA's don't have the resources or inclination to police Part P. However the crunch may come when you try and sell the property without a required Building Control Compliance Certificate.

Part P was watered down considerably a while ago. Briefly, as it now stands only new circuits, replacement consumer units or additions and alterations in areas of increased risk require notification. Areas of increased risk are now limited to the "zones" round baths and showers or in rooms containing a swimming pool or sauna heater.

No need to notify "extension leads" that supply an outbuilding. 

Good practice still comes into play. If you are burying the cable it needs to be done safely.

[hughgilhespie]

Hi,

I ran a 2.5 mm SWA to my dome and did the wiring myself, then had it checked by a genuine electrician about 6 weeks ago. He said that if there is no water in the observatory, then I didn't need a separate earth rod and eathing at the consumer unit in the house is fine. The only changes he made to my installation were to add an isolator breaker for the lighting circuit. Apparently this is in the regs. He then signed it all off for me, so I was very chuffed!

I did use decent quality kit for the wiring and all the junction boxes, switches and socket outlets were IP65 rated for outdoor use. All the wiring was in 20 mm plastic conduit.

Regards, Hugh

[Gina]

As a electtician i would earth the SWA at the consumer unit but insulate it

at the obs end and put a earth rod down at the obs end with a RCD at the obs end

Exactly what I've done except that the feed to the obsy is run off a 13A plug and RCD.

[wxsatuser]

Hi,

I ran a 2.5 mm SWA to my dome and did the wiring myself, then had it checked by a genuine electrician about 6 weeks ago. He said that if there is no water in the observatory, then I didn't need a separate earth rod and eathing at the consumer unit in the house is fine. The only changes he made to my installation were to add an isolator breaker for the lighting circuit. Apparently this is in the regs. He then signed it all off for me, so I was very chuffed!

I did use decent quality kit for the wiring and all the junction boxes, switches and socket outlets were IP65 rated for outdoor use. All the wiring was in 20 mm plastic conduit.

Regards, Hugh

If there are no extraneous conductive parts in the obsy 2.5mm is ok.

If there are extraneous conductive parts then they need bonding, so 6mm SWA cable will be needed as a minimum.

[hughgilhespie]

Hi Mike,

I used to have an extraneous conductive part but it seems to have dropped off -- the joys of ageing!

Regards, Hugh

[Alien 13]

You could introduce a seperate "signal" earth at the obs end as a refrence for any LV ground connections it should reduce any interference/noise problems.

Alan

[Ptarmigan]

No need to notify "extension leads" that supply an outbuilding. 

Good practice still comes into play. If you are burying the cable it needs to be done safely.

As an ex- IEE CEng , qual,,, no I wont bore you with all the details,

"temporarily are used in and near outbuildings" and elsewhere on your patch for lawnmowers and hedge trimmers ( oh lets not go there ! ) etc.

pay attention to Paul, for he speaketh the truth !

It is known as the law of unitended consequences,

extension leads are thus made legion.

but yes, please dont bury it !

[Scorpius]

I'm not an electrician but I do work in a US building inspections dept. as a plumbing/mechanical inspector so I’ve learned a good bit about A/C wiring by osmosis. Plus, I’ve done a fair amount of DIY wiring over the years since hiring an electrician can be a somewhat expensive proposition. It's a little confusing hearing you guys use terminology from across the pond since most of the terms over here are different. Case in point - in the US - earth is called ground.

So anyway – it sounds like you plan to install a single 110 volt branch circuit to the obsy in which case a separate grounding system is not required or permitted by US codes. This is because the circuit is already grounded through the equipment grounding conductor in the wire from the house and the whole system needs to be grounded at the same electrical potential. You shouldn’t use a “flying plug” though - the obsy circuit should originate at a properly sized circuit breaker installed in an empty space in the existing main panel for the house which hopefully is already properly grounded.

The key is to run a large enough wire to handle the peak demand and make sure the breaker in the panel is no larger than the rating of the wire. In the US, a #12 AWG is rated at 20 amps. If your load would ever exceed 20 amps you should consider installing a main lug subpanel instead of a branch circuit which would require 3 conductor cable w/ground (4 wires from the house) and all grounds and neutrals in the subpanel would need to be isolated (grounds on one bus bar and neutrals on another with no bonding strap connecting them together inside the subpanel). The advantage of a subpanel is it allows you to split the load among multiple curcuits and the higher demand equipment can be placed on separate circuits to balance the load. When going the subpanel route – the need for a separate grounding system at the accessory structure (obsy) has always been a source of debate. I believe the current requirement is for a separate grounding system if there are any metallic systems (piping, etc.) in the accessory structure but that’s only for a subpanel not a simple branch circuit like you are describing.

I can find out for sure if you decide to install a subpanel instead but for a single branch circuit to an accessory structure - fed from the main panel - separate ground rods are not required or permitted by US codes. Just make sure your cable has three wires, meaning a hot - a nuetral - and a ground

[m1dlj]

As a electtician i would earth the SWA at the consumer unit but insulate it

at the obs end and put a earth rod down at the obs end with a RCD at the obs end

I also agree. My garage/radio shack is supplied this way, based on recommendations from other radio amateurs.

Our property like many others, has a TN-C-S or Protective Multiple Earthing arrangement, i.e. only a live and a neutral cable are supplied to the house, the earthing arrangement inside the property is connected to the incoming neutral at the consumer box, and the network distributer makes sure that neutral is kept at earth potential on their side.

Theoretically, I believe there could be some risk involved if this earthing arrangement is 'exported' to an out-building, and a fault occurs somewhere in the neutral cable. Although the risk may be statistically small, it's considered sufficient so as to prohibit extending a PME system to a caravan or boat.

So the SWA cable is connected to a circuit breaker and to earth in the consumer box. The other end has the SWA is terminated and insulated 250mm short of an RCD in the shack. There is a short run of 16mm csa earth cable from the RCD to a 1 metre earth rod, and all rigs/instruments and conductive surfaces are bonded to it, so the shack has its own TT earthing arrangement. This also helps to keep the HF noise to a minimum.

It has been informally checked by an electrician (mate), perhaps I should inform some one who deals with part P regs, but then again I'll probably just disconnect it if we sell up.

I did run with an (over ground!) extension lead for a while, but it proved impractical in dark/rainy/dog conditions.

Whatever you end up with in the obsy, make sure its safe.

Good luck M1DLJ 

[kirkster501]

As a electtician i would earth the SWA at the consumer unit but insulate it

at the obs end and put a earth rod down at the obs end with a RCD at the obs end

Hi, what do you mean insulate it at the obs end?  So I agree, Earth the SWA at the house end for sure.  What would be done with the SWA shield at the Obs end?  Would it just "float" with the house end doing the Earthing? 

I will have a two way cosumer unit at the obs end.  I was just going to take the three wires out of the SWA and then they woudl be the "feed" for the obs consumer unit with the SWA grounded by the house's Earth?

[mbhyde]

Hi, what do you mean insulate it at the obs end?  So I agree, Earth the SWA at the house end for sure.  What would be done with the SWA shield at the Obs end?  Would it just "float" with the house end doing the Earthing? 

I will have a two way cosumer unit at the obs end.  I was just going to take the three wires out of the SWA and then they woudl be the "feed" for the obs consumer unit with the SWA grounded by the house's Earth?

The earth from the house should not be connected to the obs  end . If you have a pvc cosumer  unit it can be made of on it . But a earth rod will need to be put down at the obs end for earthing . The swa is earthed from the house and the obs is earthed from the rod.

[JamesF]

The earth from the house should not be connected to the obs  end . If you have a pvc cosumer  unit it can be made of on it . But a earth rod will need to be put down at the obs end for earthing . The swa is earthed from the house and the obs is earthed from the rod.

I assume this is because the house and obsy cannot be assumed to have the same earth potential, so if you shorted the house earth to the obsy structure you might actually get a current flowing?

James

[Paul M]

Where the house is supplied by a TNC-S (PME) supply the supply neutral is used as the earthing conductor also.

No big deal more often than not the neutral is earthed at the substation anyway. There may also be numerous points at which the supply neutral is connected to true earth in the distribution network also. Particularly where TN-S has been converted to PME (best of both worlds!!).

Where there is a fault or heavy loads it is possible for the neutral conductor and therefore the local supply Main Earthing Terminal (MET) to develop a voltage relative to true earth, that is the ground you are stood on. It's possible therefore that you could get a shock if you touched an earthed conductive item. The risk is minimised by creating an equipotential zone in the home by way of main bonding and supplementary bonding.

Outbuildings are by definition outside the earthed equipotential zone and so the domestic earth arrangement should not be exported. In such circumstances all exposed conductive parts should be bonded to a local and dedicated earth rod.

Less of a risk with TN-S supplies. The old lead sheath cables that usually provide that are usually buried directly in the ground so ensuring a very good reference to true earth.

In this age of RCD's no one seems to care any longer. TNC-S earths are routinely exported in my area.

It isn't a purely academic issue. There was a recent incident where a housing terrace suffered multiple fires due to a supply neutral fault. The whole network neutral current was finding it's way back to the transformer via the domestic earth bonding conductors. They were glowing red hot and popping like fuses. 

[kirkster501]

The earth from the house should not be connected to the obs  end . If you have a pvc cosumer  unit it can be made of on it . But a earth rod will need to be put down at the obs end for earthing . The swa is earthed from the house and the obs is earthed from the rod.

Ok just to recap.  What you are saying is (?)

1. The SWA metallic sheath is Earthed at the house end.

2. The SWA metallic sheath at the obs end is NOT earthed there - can be terminated on the plastic of the PVC consumer unit for neatness

3. The Earth for the plugs etc INSIDE the Obs does not use the Earth fed from the house but uses a "local" Earth inside the Obs which is provided by means of a Earth Rod.

Assuming [3] is true why is 3 core needed to be run from the house then?  Why not just run Postive and neutral?

[JamesF]

It isn't a purely academic issue. There was a recent incident where a housing terrace suffered multiple fires due to a supply neutral fault. The whole network neutral current was finding it's way back to the transformer via the domestic earth bonding conductors. They were glowing red hot and popping like fuses. 

That's something I'd not thought of, I have to admit.

James

[Alien 13]

Fom an electronics point of view most equipment nowdays is double insulated and does not use an earth (bad move) so having a local earth at the obs is a sound solution a problem could however arise if you had 50 foot long arms and where able to touch the house earth and the obs at the same time.

Alan