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Topic Title: the old sup bonding chestnut
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Created On: 29 October 2017 04:16 PM
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 29 October 2017 04:16 PM
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Zs

Posts: 3827
Joined: 20 July 2006

Hello,

A large non- electrical piece of equipment, bolted to the floor is to be connected to the main earthing terminal.

The environment is three phase.

A couple of the requirements of the spec are causing me a bit of worry.

Now, amongst friends, you know that this is my weak spot. so I'd like to share my concerns with you.

This first one I have already stuck my neck out on and it's pretty easy.. they asked for a memo about how to test the continuity of the parts of the equipment in step by steps. (Fills you with confidence no doubt ) So, it says that when confirming operation of the test meter, hold the two probes together and make sure that it says 0 ohms and Zero the meter. It then goes on to describe the test method.

that came back changed to < or = 1 ohm.

Then, elsewhere in documentation is the one I'm hesitating to address but it says that a reading between metal parts of the equipment and the MET must be 10 ohms or fewer. I put it in as 0.05 originally. Have you any idea where the 10 number comes from?

Going through all my notes, GNs, Wiley books and sketches by OMS from the years, I don't see it anywhere. But before I speak out incorrectly, and working on the basis that If this is an extraneous-conductive-part by dint of some rebar in the concrete floor for example, then touch voltage applies?

I don't get it. Could you please confirm or confound my misgivings?

thank you,
Zs
 29 October 2017 05:57 PM
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Fm

Posts: 1733
Joined: 24 August 2011

What's the floor made of?
What can you touch while touching this new equipment?
 29 October 2017 06:02 PM
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geoffsd

Posts: 1760
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Originally posted by: Zs

hold the two probes together and make sure that it says 0 ohms and Zero the meter. It then goes on to describe the test method.
that came back changed to < or = 1 ohm.

The leads will have some small resistance so will not be zero, otherwise there would be no need to zero the meter.
The 1 ohm can only be an arbitrary figure. My Fluke leads read 0.28 ohms.

Then, elsewhere in documentation is the one I'm hesitating to address but it says that a reading between metal parts of the equipment and the MET must be 10 ohms or fewer. I put it in as 0.05 originally. Have you any idea where the 10 number comes from?

No, I don't know where that comes from.
Nowhere is there a limit or recommendation for the overall impedance of main bonding conductors.
10 ohms seems a lot. That's about 5,500 metres of 10 sq.mm. conductor?

Supplementary bonding depends on the circumstances.

The 0.05 ohm often quoted is just an accepted value for negligible impedance of the bonding connection itself.
 29 October 2017 07:16 PM
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AncientMariner

Posts: 718
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I think that the 10 ohms should have been 1.0 ohms.

I have nothing to back that up, just my gut feeling.

Clive

-------------------------
Clive S Carver GCGI IEng MIET MITP
 29 October 2017 07:24 PM
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sparkingchip

Posts: 10107
Joined: 18 January 2003

I suspect you have Already read this or similar articles

If the equipment is non-electrical is it supplementary bonding or a main protective bond?

Andy B.
 29 October 2017 07:31 PM
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Zoomup

Posts: 3366
Joined: 20 February 2014

Yes, many multimeters allow us to "zero" the meter before carrying out resistance tests. The leads will have a certain resistance, but the meter can often be set to zero Ohms by using a rotary knob to read zero Ohms with the leads shorted together. That way the lead's resistance is automatically cancelled out and the actual readings will be accurate when the meter is in use. More modern meters have an auto zero function I believe.

Z.
 29 October 2017 07:41 PM
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Zoomup

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Is the non-electrical equipment an extraneous conductive part? If not it will not need bonding. It is just a lump of metal standing on a floor. But it may be an extraneous conductive part if it has metal bolts holding it to a concrete floor. Why not carry out an insulation resistance test between the metal equipment and the main earthing terminal at 240 Volts to see what the resistance is?

Z.
 29 October 2017 07:50 PM
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paulskyrme

Posts: 1291
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So Zs,


Do they bond metal stairs and other things in this location still?

So, this is a hunk of metallic equipment without an electrical supply yes?

Firstly, we use bonding to equalise the potential between exposed metallic parts in the installation yes?
Many of which are already connected to the general mass of earth via the electrical installation, i.e. the circuit protective conductors of their respective supply circuits.
So we ensure that they are reliably connected so that no potential can exist between these and true earth in the event of a fault condition.

Next, we bond extraneous conductive parts in the installation.
These parts introduce a potential into the installation that would not otherwise be there, or they introduce another instance of a potential, generally earth potential into the installation.

So, is this equipment connected to the general mass of earth?

Does it import a potential, generally earth potential into the installation?

What is the resistance (impedance) between the earthed equipotential zone already established within the building and the metallic equipment?

Would bonding this equipment, brining it to earth potential with negligible impedance to true earth, increase the hazard caused by the equipment?

As far as the test procedure goes, I'm sure it's a typo in your post, you say that the reading should be zero ohms before "zeroing/nulling" the meter, which it would never be.
I think you meant to say, connect the leads together, then zero the meter, and then check that the reading is 0 Ohms. ??
I think that they have changed this to allow for up to a 1 Ohm resistance in the test meter leads.
This IMHO is excessive.
Any set of leads with>> 0.5 Ohms resistance for a "normal" set of DMM or MFT test leads I would consider excessive and be looking at the lead set being potentially faulty.

I see that there is a link to the WM article by Mark, which is obviously relevant, and I don't recall containing any errors.

Have a look in the guidance given in GN8, I have emailed you some other stuff in case it helps.
 29 October 2017 07:50 PM
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Zoomup

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Doesn't GN8 suggest that if the resistance between the conductive part and the main earthing terminal is above 22kOhms then it will not need bonding, as it is not considered to be an extraneous conductive part.

Z.
 29 October 2017 07:51 PM
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sparkingchip

Posts: 10107
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I assumed it is considered "earthy" due to the fixing bolts, having said that I didn't bond the chair in a hairdressing salon after I bolted it down to the floor, though unless they skimped on the concrete it should be all above the damp proof membrane.

Andy B.
 29 October 2017 08:00 PM
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sparkingchip

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I also assumed the request to confirm the test method was to establish the outcome of the test, not how to null the meter.

 29 October 2017 08:31 PM
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OlympusMons

Posts: 65
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I am assuming that there is an earth terminal on the equipment, perhaps the exposed metal parts have a maximum designed resistance of say 9 Ohms to the earth terminal, leaving say 1 Ohm for the supplementary bonding cable.
We don't know what the equipment is, but it could be building up static electricity during use which might need to be grounded.
 29 October 2017 08:32 PM
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leckie

Posts: 4397
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Well I see specifications from consultants for reasonable size jobs and part of the specification often includes requirements to supplementary bond all HVAC equipment, cable tray, cable ladder, general lumps of metal. Also a requirement to measure the earth continuity of cable ladder, tray, etc., and record and document the measurements. But the tray and ladder, metal conduit are not to be considered as CPC's, separate CPC's are required for each circuit.

So what is the consultant thinking of when making these requirements part of the specifications? Let's assume that the lumps of metal are not extraneous conductive parts - they are still requiring that they are supplementary bonded. Everything made of metal.

This is on lots of specifications I am seeing. Bonding to all big lumps of metal and supplementary bonding between the lots.

So what is their reasoning do you think?
 29 October 2017 08:54 PM
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geoffsd

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A large non- electrical piece of equipment, bolted to the floor is to be connected to the main earthing terminal.

Main Bonding.
 29 October 2017 09:01 PM
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leckie

Posts: 4397
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But if the floor is insulated from terra firma, is there a need to main bond lumps of metal bolted into that floor? Bit different if the metal is bolted to earthy lumps of concrete, etc.
 29 October 2017 09:11 PM
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geoffsd

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We don't know.

Zs could ask them if they are certain it is necessary - and possibly educate them.
 29 October 2017 09:15 PM
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UKPN

Posts: 667
Joined: 17 January 2012

Geoffsd, is correct with his comment on main bonding, we would want to see that done. As regards the 10 ohm query that is an old figure used by the electricity boards for staking down the PME network. Its 20 now, how the 10 crept in this situation we probably never know. There is no figure reqd by the DNOs for resistance of main bonds, there was many years ago when the sizes were tiny, ie 2.5mmsq. Nulling the leads? should be easy enough with a separate tester if in doubt.

Regards, UkPN
 29 October 2017 10:16 PM
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sparkingchip

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Don't mention nulling the leads, have you not realised that part of the discussion raises doubts about competence and understanding, partly due to its inclusion in the reply to the request for clarification and partly due to the inaccuracy of reply? The poster doesn't need to be told how to null the leads!

 29 October 2017 10:23 PM
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sparkingchip

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Originally posted by: sparkingchip

Don't mention nulling the leads, have you not realised that part of the discussion raises doubts about competence and understanding, partly due to its inclusion in the reply to the request for clarification and partly due to the inaccuracy of reply? The poster doesn't need to be told how to null the leads!





Which may suggest it was a sarcastic reply, which went belly up.

Andy B.
 29 October 2017 10:32 PM
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sparkingchip

Posts: 10107
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We don't know what and where this lump of metal is, however I suspect the discussion needs to head in this direction.

Andy
IET » Wiring and the regulations » the old sup bonding chestnut

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