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PostPosted: Wed Aug 19, 2015 7:42 pm 
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Location: Sleaford, Lincolnshire, UK
Hi all,

I’m currently designing the construction of my (UK) home ‘music room’, see here, and am struggling understanding the issue of grounding to avoid potential ground loop hums. I have spent hours reading the brilliant threads in this category and there’s obviously a lot to absorb – so much so, I now cannot see the wood through the trees. Hopefully this fresh thread will be of interest to the little man, like myself, who just wants to modify his adjacent space into a modest project studio, and hasn’t the lifespan left to learn the British Standards by rote before he dare lift a screwdriver. With the greatest of respect and gratitude to the former contributors, please may I request a practical condensation of this?

So, my mains power to my room comes in from a 10mm2 cable from a breaker on the household distribution board (PME, no local ground rods), to a separate sub-distribution box (SDB) inside the music room. Then, from its own circuit breaker in this SDB, I intend to wire the wall mains sockets (U.S. receptacles) - exclusively for my audio equipment, reckon about eight, on a ring – i.e. LNE cable, chained from socket to socket, both ends resolving at the sub-distribution board. No (metal) conduit, just 2.5mm2 twin and earth clipped to the walls (common-sense straight lines up/down, across, top bottom, etc.), all behind a drywall lining.

Appreciate how the safety side of things are primary yet a parallel but separate subject. But purely for sound, does this mean I should take a separate heavy gauge earth wire from each socket’s ground pin back (run parallel to existing wiring) to one common, nearby but separate point (a copper bus bar, for example), then connect one final wire from this bus bar into the earth terminal into the SDB?

Do I then also connect the safety earth conductors in the chained ring cabling to the sockets’ back boxes, but just not link the back boxes to the socket’s ground pins – obviously connecting these safety earth wires back at the sub-distribution box?

Will it matter that the additional star grounding conductors will be of different lengths back to the bus bar, or should I try to keep them all the same length to match impedance?

Will plugging in further multiway plug extension strips into the wall sockets undo all of the above, and if so, should/could they be modified in any way?

What about having a separate star-ground earth conductor from the common point, routed to another bus bar/terminal near the console (say), that I could earth any misc. equipment to test/solve temporary hums?

Finally, please don’t panic. I intend to have the above wiring done (or at least inspected) by a qualified electrician.

Thanks

Kevin


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PostPosted: Wed Aug 19, 2015 9:57 pm 
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Location: Cleveland, Ohio, USA
Forget the 'star ground' idea, it won't work with a 'ring' circuit. The 'ring' should consist only of a Hot, Neutral & Protective Earth, all in a one-to-one-to-one one relationship. The three conductors should be close to each other at all times.
I don't know what your ' 2.5mm2 twin and earth' looks like but it should have a gentile twist or spiral from end to end.
Don't worry about a stake in the garden, just do whatever the electrical code requires.

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PostPosted: Wed Aug 19, 2015 11:15 pm 
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Hi Speedskater, thanks for your prompt reply!

I'm surprised at your very terminal advice however. Ring circuits in the UK are in every home since time began almost, and 2.5mm2 (or whatever sizes) twin and earth is what is/has been used for several decades, I assume. Sadly it's not twisted (I assume for EMC/RFI benefits?), but it's a two sleeved live and neutral, and an unsleeved earth, all surrounded in a flat, parallel insulation.

Ring circuits are a effective method of domestic mains distribution on a level, that distributes the loads and facilitates later modification via spurs, etc. - code compliant, of course. There must be studios across the globe that have their mains distributed on a ring circuit, surely?

You know all this, right? :?

I thought I said in my OP that I didn't need further earth stakes?

With respect, please would you explain why I should forget about star grounding? Are you suggesting wiring it radially instead??

Thanks again,

Kevin


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PostPosted: Tue Aug 25, 2015 8:26 am 
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Hi Kevin,

Some quick thoughts:

a] Your 'twin-and-earth' is very similar to our Romex®. While it would be good if just the Live and Neutral were twisted (yes, for EMI/RFI benefits). But they are not, so we can twist the entire flat cable (maybe 1 or 2 twists per foot).

b] I didn't mean to suggest that you had added earth stakes, but for other readers, a common myth is that added stakes (often not connected to the main Earthing system) were good for AC power quality.

Now getting back to 'Star Grounding'.

a] A 'Star Grounding' system is the exact opposite of a 'Ring Circuit'. Remember that the Live, Neutral and Protective Earth all have to follow the same path.

b] A 'Star Grounding' system is the same as a 'Spur Circuit'.

Now as to whether to use a Ring or a Spur. I would chose the one that has the shorter wire lengths between your components.

'Ring Circuit ' systems may be limited to the UK.

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PostPosted: Tue Oct 20, 2015 8:56 pm 
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Hello again, Speedskater. Thanks for your reply - I'm sorry, I appear to have missed it when first published.

Copy the trick of twisting the T/E to act as a transformer against RF currents. No earth stakes.

For info, only one spur is allowed from each socket in/on a ring circuit in UK circuits I believe.

In my tiny project room, wiring radially from the sub-board is just as easy - albeit, perhaps more cable than a ring, and to my perception, would mean each socket's equipment/audio earth(s) would go back to a common bus in the sub-board - along a chunky additional earth conductor - routed alongside the T&E/Romex. (T&E=Twin and Earth)

Perhaps I'm over-thinking it then, trying to keep the earth pin from the equipment/receptacles 'separately' commoned at the sub box (by the additional single conductor), but then keep the recepticle's frames and corresponding T&E/Romex supply cables also commoned back at the sub-board, but thus protected against shock e.g. an earth fault inside the receptacle, or someone accidentally damaging the T&E/Romex by drilling, etc.

Just want to do all I can to rule out buzz/noise, before I dry-line my walls. So my last request is acknowledgement that you get what I am trying to do, that it's not dangerous, and that it's not a complete waste of time and money. If contrary, what would you do if you were (crazy enough to) convert a garage into a music room/mini studio, and was about to route your mains supplies?

Thanks again

Kevin


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PostPosted: Wed Oct 21, 2015 2:52 am 
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But a Twin & Earth (Romex®) shouldn't have another Protective Earth added.

Any circuit, Ring or Spur should have:
A Hot, Neutral & Protective Earth, in a one-to-one-to-one ratio, all in close proximity to each other.
The conductors all should start in the same place. With the ring, end in the same place.

*******************************************
In the US system we often see stars or multiple stars.
But the star is for all 3 conductors H, N & PE.

So one cable (H, N & PE) leaves the breaker box and travels to one junction box. At the junction box, it splits into 2 or more cables that may go to more junction boxes or outlet boxes.

*****************************
A good layout would reduce the length of cables & cords from audio component to component.

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PostPosted: Wed Oct 21, 2015 3:24 am 
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This is the part that I don't understand:

and to my perception, would mean each socket's equipment/audio earth(s) would go back to a common bus in the sub-board - along a chunky additional earth conductor - routed alongside the T&E/Romex. (T&E=Twin and Earth)

Perhaps I'm over-thinking it then, trying to keep the earth pin from the equipment/receptacles 'separately' commoned at the sub box (by the additional single conductor), but then keep the recepticle's frames and corresponding T&E/Romex supply cables also commoned back at the sub-board, but thus protected against shock e.g. an earth fault inside the receptacle, or someone accidentally damaging the T&E/Romex by drilling, etc.


**************************
Romex® has a plastic covering, insulation, sheath. We have other systems that involve a ridge or flexible metal covering. These systems sometimes use special Isolated Ground receptacles. So this IG system has an added PE/SG conductor.
***********************
Back to you idea. How do you connect both the T&E (PE) and your conductor (PE) at the receptacle?

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PostPosted: Fri Oct 23, 2015 12:03 am 
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I meant the T&E goes from in box to the first receptacle in the ring, but its PE doesn't go to the earth pin, just to the back box (stud) and then connected to the continuing ring cable's PE, and so on.

Another separate earth conductor from the receptacle earth pin then returns to the box, as one does from each earth pin on each receptacle in the ring. Both T&E earth conductors, and the additional earth (pin) conductors terminate in the box (or a separate adjacent bus bar bonded to the box's internal earth bus).

All this confusion feels wrong. I appreciate your continued interest, Kevin, but I'm keen to get on with the wiring, so I'm going to play it safe and wire it as a ring, as per code, and worry about hum later - if indeed it ever occurs!

Thanks again,

Kevin


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