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#144077 - 10/04/05 03:16 AM Alan's Workshop Earthing
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Topic split from RCD/Cooker thread:

 Quote:
HELP!!!! This thread has given me a real headache!

Wolfgang's revelations about the effectiveness of earthing via a simple rod have led me to realise that my three-phase antique woodworking machines, run off a generator, have inadequate protection against earth faults/shock.

The alternator is wired star, and makes around 400v 50hz. A full description has been been posted, (see Photos for Discussion 5-18-05 ), and at that time I re-wired centre star to earth on advice from the Forum. There's about 10 metres max. between the alternator and the furthest machine, and I have a 10A 3ph breaker as overload, which see. All machines are earthed using a discrete shop 4 ft copper rod into heavy clay.

Ideas anyone?

Alan
Oh, and thanks Wolfgang!

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#144078 - 10/04/05 03:25 AM Re: Alan's Workshop Earthing
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
If I'm interpreting this correctly, each machine is earthed separately by its own 4 ft. rod.

The least amount of work would probably be to install an RCD at the generator output to provide earth-fault protection to the whole lot.

Personally, I'd rather go with running earth conductors to each machine along with the phases and neutrals (thus creating a TN-S system), although obivously that's going to involve a lot more work re-running cables.

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#144079 - 10/04/05 11:55 AM Re: Alan's Workshop Earthing
Wolfgang Offline
Member

Registered: 09/25/05
Posts: 154
Loc: the very West of Germany
I was asking for location 'cause all receptacles were British.

At the low maximum current out of the small genny the magnetic part of a normal breaker might not work at all even in a TN-S system. In this case in Germany you will install an RCD at any rate "as maximum short current is not sufficient" for fast tripping of breaker.
Of course I would recommend a TN-S system for this application.

A separate earthing of every machine in one workshop will be a "code violation" in German eyes.

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#144080 - 10/05/05 10:57 AM Re: Alan's Workshop Earthing
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
I suppose this should be posted here instead of in the other thread.
Wolfgang,
I don't see where you get the high figure of 50 ohms for the total impedance of a Fault loop from?.
Over here in New Zealand, the maximum fault-loop impedance for an Installation using a Main fuse size of 63A, would be 0.55 ohm.
Where this device is a circuit-breaker, the maximum would be 0.49 ohm.
This would provide the required 0.4s maximum disconnection time.
Even assuming the installation is protected by a fuse and at a maximum 5s disconnection time the maximum loop impedance would only be 0.94 ohms.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#144081 - 10/05/05 01:22 PM Re: Alan's Workshop Earthing
Wolfgang Offline
Member

Registered: 09/25/05
Posts: 154
Loc: the very West of Germany
@ Trumpy

Then you simply do not have a TT system. TT means that local earth electrodes and local PE ssystem is not bonded with (European neutral/grounded conductor. So is case of fault f.i. hot wire touching metal body, the current loop is via PE, local ground electrode, ground, transformer electrode, maybe transformer grounding impedance, to star point/neutral of transformer.

Your values are typical values for TN-system where PE and neutral are bonded f.i. at the entrance of service to house. That what I have at home in Germany.

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#144082 - 10/05/05 02:41 PM Re: Alan's Workshop Earthing
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Yep, with a TT system the impedance can be much greater, but why it's practically essential for all TT systems to have RCD protection for the whole installation.

As I understand from what Mike (Trumpy) has said in the past, all services in New Zealand are wired as TN-C-S (or what you might call the PEN system in Europe).

Personally, I'd be quite happy to see TT thrown on the scrap heap entirely. It's rather interesting to note that the American NEC doesn't allow TT earthing at all. Maybe that should tell us something?

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#144083 - 10/05/05 05:42 PM Re: Alan's Workshop Earthing
aussie240 Offline
Member

Registered: 07/06/05
Posts: 222
Loc: Blue Mountains, NSW, Australia
 Quote:
the American NEC doesn't allow TT earthing

Probably because it would be harder to get it to work reliably with only 120V.

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#144084 - 10/05/05 10:46 PM Re: Alan's Workshop Earthing
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8540
Loc: SI,New Zealand
Paul,
 Quote:
As I understand from what Mike (Trumpy) has said in the past, all services in New Zealand are wired as TN-C-S (or what you might call the PEN system in Europe).

Thanks for that mate.
Yeah,
As far as to the Main Switchboard is concerned, we have the MEN (Multiple Earthed Neutral), where the Neutral is Earthed at the Xformer and at the Installation end.
(That's where Inspection ends)
However, some Sub-Mains here can use some pretty wierd set-ups, especially out in the more rural areas that have probably never seen an Electrician in 50 or more years.
I've learned to expect the absolute worst in places like this.
If I'd had a digital camera about 5-10 years ago, there would be a seperate Non-US Violations Area here.
I'm not joking!.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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