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Earth Bonding in a PDU

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Earth Bonding in a PDU

Postby nhirst » Sun Jan 20, 2019 11:33 pm

Hi,

Just picked up an Olson 2676 rack PDU to help with studio wiring, and have encountered an external earth stud that I'm not quite sure what to do with.

Do I need to wire this in a particular way? Guidance on base of unit is to bond to something metal and ensure that is connected to ground, but nothing in my studio is going to be earthed in that way.

Main worry is safety - although introducing hum would be annoying. I only bought this (cheap on eBay) as a way of keeping wires neat...

Product is here:

https://olsondirect.co.uk/10-way-19-1u- ... t-pdu.html

but the one I have only has a threaded stud on the rear - there's no earth wire coming off it.

Cheers.

N
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Re: Earth Bonding in a PDU

Postby James Perrett » Mon Jan 21, 2019 2:19 am

Is there continuity between the case of the PDU and mains earth?
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Re: Earth Bonding in a PDU

Postby nhirst » Mon Jan 21, 2019 9:20 am

not sure, frankly. probably not. the unit will be racked in a wooden enclosure. other units in the same rack are wired with a regular ole uk plug which may have earths (I'd need to check). but other than that no.

forgive my noob-level electronics knowledge.
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Re: Earth Bonding in a PDU

Postby Sam Spoons » Mon Jan 21, 2019 10:55 am

I think JP means "is the earth pin on the male IEC/Mains plug connected to the case of the unit rather than the case connected to the rack"? (which may introduce earth loops between multiple pieces of rack gear). You'll need a continuity tester of some kind or a multimeter (or a PAT machine). The mains earth will definitely be connected to the earths on the output sockets but may not be connected to the case.
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Re: Earth Bonding in a PDU

Postby Hugh Robjohns » Mon Jan 21, 2019 11:19 am

nhirst wrote:Do I need to wire this in a particular way? Guidance on base of unit is to bond to something metal and ensure that is connected to ground, but nothing in my studio is going to be earthed in that way.

As others have said, you need to check first whether the case of the PDU is bonded to the earth pin of the mains inlet. If it is, then you can leave the earthing stud alone...

...but there's a possibility that it isn't...

In which case you should arrange for the case to be separately safety grounded. In your case, with a wooden rack frame, I'd suggest the most practical solution would be by wiring an earth cord from the stud into the earth pin of a UK mains plug, and plug that into the wall socket alongside the actual power inlet cable to the PDU.

For safety, that casework needs to be properly grounded, one way or another. Conventionally, it would normally be through the metal rack frame.

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Re: Earth Bonding in a PDU

Postby Wonks » Mon Jan 21, 2019 1:13 pm

I feel that the external earth stud is simply for equipotential bonding of exposed metal parts as required by IEE regs (just bolting the PDU into a metal rack isn't considered sufficient, so dedicated earth straps are required).

The case should already be connected to the incomming plug ground. It never hurts to check with a meter but you should be able to visually confirm it.

The PDU is really designed for fitting in 19" metal rack cases in server rooms etc. and the PDU stud is the means by which the rack itself is safety grounded.
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Re: Earth Bonding in a PDU

Postby nhirst » Mon Jan 21, 2019 6:19 pm

Hugh Robjohns wrote:
As others have said, you need to check first whether the case of the PDU is bonded to the earth pin of the mains inlet. If it is, then you can leave the earthing stud alone...

...but there's a possibility that it isn't...

Is that something I can do easily with a multimeter? Or is it best left to a pro...?

Hugh Robjohns wrote:In which case you should arrange for the case to be separately safety grounded. In your case, with a wooden rack frame, I'd suggest the most practical solution would be by wiring an earth cord from the stud into the earth pin of a UK mains plug, and plug that into the wall socket alongside the actual power inlet cable to the PDU.

For safety, that casework needs to be properly grounded, one way or another. Conventionally, it would normally be through the metal rack frame.

This is starting to sound like a job for an electrician...

Did I buy a stupid thing?
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Re: Earth Bonding in a PDU

Postby Sam Spoons » Mon Jan 21, 2019 6:35 pm

Do you have a multimeter? If not it's worth getting one and learning a few basics, very useful for anybody who uses electric instruments or studio gear. If you are not confident then, yes it is a job for a competent techie (or, indeed, an electrician) but it is not at the difficult end of electrical testing/faultfinding and even a cable tester with a continuity beeper will do the job of checking the earth between the plug and the chassis.

And no, you didn't buy a stupid thing :thumbup:
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Re: Earth Bonding in a PDU

Postby Hugh Robjohns » Mon Jan 21, 2019 6:55 pm

Sam is quite right, you didn't buy a stupid thing, and it is a very simple thing to check out with a multimeter yourself.

Just set it to the resistance or continuity mode, attach the mains inlet cable to the PDU, place one probe on the earth stud and the other on the earth pin of the mains plug. You should get a reading of an ohm or two. While you're there, take the probe off the stud and slip it into earth contact of the output sockets too.
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Re: Earth Bonding in a PDU

Postby Wonks » Mon Jan 21, 2019 7:04 pm

As I've said, the PDU is used to ground a metal rack, not the other way round. Power, including the earth, is first fed to the PDU.
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Re: Earth Bonding in a PDU

Postby Sam Spoons » Mon Jan 21, 2019 7:44 pm

FWIW the Olsen website says of the 2676 PDU :- " • Earth bonded with external M5 earth stud fitted with 250mm earth lead and terminated with a M6 earth tag"
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Re: Earth Bonding in a PDU

Postby ef37a » Tue Jan 22, 2019 7:21 am

Bit worried by, " but nothing in my studio is going to be earthed in that way."

So, in what way IS gear in your studio going to be earthed N? The incoming mains earth must be bonded to the PDU's case by law but yes, DO check that for a second hand unit and should it not be, open it up and see why. If there is evidence of amateur "customizing", maybe the previous incumbent had earth loop problems, see about getting a refund.
But I think that is unlikely and as others have said, the earth stud is to bond other gear in a rack*.

Note, even with a wooden case there will be metal rails and these can earth, often badly and intermittently, the various rack units. You can buy "Rack Snaps" that insulate each gear's case from the rails and thus each other. Of course, ALL gear that needs an earth must still get one and in this case that will be the PDU.

And YES! Get a test meter! If you just had a mic, AI and cans maybe not but for anything slightly more involved you need one.

*Not sure the earth stud is intended for earthing the rack frame? I have built one or two up for comms rooms and there is a separate earthing stud for a heavy duty, dedicated earth. In shielded network systems some large earth difference currents can flow. In any case the earth for the rack would not be left to the whim of a gash 13A mains lead!

Just in case and if I am insulting you? Don't care in this instance. When I said "open it up" I of course meant with the PDU completely disconnected from everything!

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Re: Earth Bonding in a PDU

Postby Hugh Robjohns » Tue Jan 22, 2019 11:50 am

Wonks wrote:As I've said, the PDU is used to ground a metal rack, not the other way round. Power, including the earth, is first fed to the PDU.

Not so sure about that. If that were the case, unplugging the mains inlet to the PSU would remove all earthing from the rack... which wouldn't be the greatest idea! Every rack installation I've ever seen has had a dedicated earth strap to it.

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Re: Earth Bonding in a PDU

Postby nhirst » Tue Jan 22, 2019 11:25 pm

Thanks all. I do have a multimeter so will check out the unit as suggested.

Confident enough to have a go, but have been warned off tinkering around with anything mains enough times to check here first!

Re earthing... I’m possibly being naive but I assumed everything else was earthed through my (UK) plugs. This is the only unit I have with a dedicated earth separate from the power input.
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Re: Earth Bonding in a PDU

Postby DGL. » Tue Jan 22, 2019 11:39 pm

From what i understand the only reason why it has the extra earthing lug is due to oversized earth cables being required in a server (or similar) rack, these being of a diameter that won't fit in a standard plug.

As has probably been pointed out already to test for earth continuity put the meter into it's lowest ohm setting and make sure that you have less than 1Ω (a PAT would usually fail if greater than 0.2Ω) between the ground/earth on the input connector/plug and all of the output connectors.
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Re: Earth Bonding in a PDU

Postby Hugh Robjohns » Tue Jan 22, 2019 11:47 pm

nhirst wrote:Confident enough to have a go, but have been warned off tinkering around with anything mains enough times to check here first!

Always wise to be cautious around mains, but in this situation earth continuity testing is a safe exercise.

I’m possibly being naive but I assumed everything else was earthed through my (UK) plugs. This is the only unit I have with a dedicated earth separate from the power input.

Class-1 devices have their chassis and any exposed metalwork earthed via the (three-pin) mains plug. Class-2 devices (also known as 'double-insulated' and marked with the box-within-a-box graphic symbol) do not. Any exposed metal work is isolated from any mains wiring/terminals by at least two insulating layers (one of which can be air).

Some pro gear has two earth terminals on the rear panel. One is an extension of the mains earth and the other is the internal audio ground. The two are normally linked together but may be separated for installations where there is a dedicated technical earth system in place.
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Re: Earth Bonding in a PDU

Postby wireman » Wed Jan 23, 2019 8:25 pm

This whole rack earthing discussion is not going to help that much in the case of a wooden enclosure. I even asked an engineer who installs computer and storage systems about this the other day and there are different situations, in the datacentre he works in some PDUs are connected to the rack and some are not.

I think anyone who purchases second-hand metal mains-powered equipment would do well to get a simple multimeter that has a low resistance scale or continuity and use it to confirm the earth pin on the power lead is connected to the outer metal casing. You can use it to check fuses as well.
I do this routinely and in fact I sent back a new lamp last year because the metal stem was not earthed.
Double insulated products tend not to be metal and would have the square in square symbol to confirm their status.
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Re: Earth Bonding in a PDU

Postby ef37a » Thu Jan 24, 2019 9:07 am

I would expect Wireman that a commercial wooden rack would be a conventional metal box clad in tree?

A home made box would still have rack strips? (you COULD screw into hardwood rails but I would not recommend it) and as part of the "external metal surface" they would still need an earth bond not at the whim of a 13A plug?

Oh and, wood is not a GOOD conductor of electricity but it ain't PTFE!

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Re: Earth Bonding in a PDU

Postby Hugh Robjohns » Thu Jan 24, 2019 12:16 pm

ef37a wrote:I would expect Wireman that a commercial wooden rack would be a conventional metal box clad in tree?

No. I think all of the ones I've seen are just (solid) wood with rack strips bolted to the insides.

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Re: Earth Bonding in a PDU

Postby Wonks » Thu Jan 24, 2019 12:29 pm

You also have the added complication that a lot of musical rack gear wants to remain isolated from the rack earth to avoid ground loops, so isolating washers are fitted.
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