Earthing regs on 19" rack gear

[charliecoutas]

Sorry, nothing to do with Positron but I know there are some Power Experts here:

A pal of mine is doing the mains wiring on a big 19" rack. Can somebody answer this please: we need to earth each individual (metal) unit with a separate earth wire, to a common earthing point on the rack, and this point must go to the supply earth.

Question 1: Does that earthing wire have to be at least the same sq mm/" as the supply to that unit?
Question 2: Does the main (3 phase) supply cable have to have a separate earthing wire? If so, what guage?

Thanks in advance.

Charlie

[Fanie]

Hello Charlie !

The earth cable does not have to be heavy for high currents like the power carrying wires, but it has to be reliable, in other words not a thin finicky wire or something that can be broken off accidentally.  You can get earth wire from any electrical outlet and usually are not isolation coated (bare).  The earth wire must make a proper electrical connection, ie a wire terminated in a lug that is screwed down on a bare metal (unpainted) surface.  Such racks usually have a special screw-in hole pre-made for just that purpose.

Usually it is expected that any supply cable will have an earth otherwise you may experience different earth levels at different places, although if there is already a supply that is earthed I'm sure you can use their earth.  The earth is only there for protection for an earth leakage.

Just take into consideration that if an earth connection is available from elsewhere that you may have to ask permission from who ever else is responsible for that installation.  They will not take it lightly if you fiddle with their installation wiring.

Most of these so called safety measures is there for stupid people doing stupid things with live wiring (in my opinion).  Nature has a way to phase (pun intended) idiots out, but it seems they want them preserved for more future idiots.  Here by us it happened that the foreign parasite working on the 380V at the substation neglected to connect the neutral back and switched the supply back on.  Half a suburb's electrical appliances burned out and the sh3ts just pull their shoulders up.

On a lighter note, since we're with appliances now,
And God said to John: "Come forth and receive eternal life".
But he came fifth and won a toaster 

One more

In the beginning, God created the heaven and the earth
After that, everything else was made in China.

[Fanie]

If someone is to inspect the equipment, they will likely pull a megger out and megger the daylights out of the frame and the supply earth, if they get a voltage the installation will fail, if it remains a short it will pass.

[DaveS]

Hi Charlie

All depends, follow IEE regs and manufacturers installation instructions

Question 1

My personal opinion is it is not necessary to install a cable between each tray section, as these will have metal joining brackets fitted at each section and bolted however manufacturers installation instructions may override common sense.

Question 1: Does that earthing wire have to be at least the same sq mm/" as the supply to that unit?

I would normally make the tray mechanically connect to the metal frame work of the unit suppling the power thus no earth cable required, if you use an earth wire see video below.

Question 2: Does the main (3 phase) supply cable have to have a separate earthing wire? If so, what guage?

I take it is a SWA cable, not always the SWA of the cable should be sufficient in most cases, certain BS numbers of SWA may not have sufficient steel c.s.a. so a separate earth is required, why make cable like that?
Basically, calculate if the cable will set fire, before the fuse blows, that's how you get the minimum size required.
https://professional-electrician.com/technical/armouring-of-a-cable-as-a-protective-conductor/
Most sparks go OTT to cover there self, earthing everything in sight

This video explains some of it
https://www.youtube.com/watch?v=SJFGoeqtr04

[charliecoutas]

Dave, Fanie

Thanks very much for all the useful info. Those videos are very helpful Dave. I'll pass this all on to my mate.

Best
Charlie

[charliecoutas]

Fanie

This picture says quite a lot?

Charlie

[Fanie]

Those I sent your picture to liked it.
I get the impression that Trumpet see how successful those are they have sanctioned and decided to sanction themselves 

[SeanG_65]

Remember that your rack MUST have an INDEPENDENT earth, and NOT just rely on the one in the power cable if it is a static setup. Each removable panel MUST have an earth wire the same sq.mm as the MAIN power wire "Live" so that it can carry the maximum fault current and ALL those panel wires MUST go to a SINGLE M10 or greater stud to which the auxiliary earth is connected.

I have designed TE for GE, TRL, TRW, RR and quite a few others. I prefer overkill on safety. It's simply NOT worth someone's life (unless a banker or lawyer).

[charliecoutas]

Many thanks Sean, you make a very important point. I'll pass this on to my friend.

Charlie

[RGV250]

Hi Charlie,
I have retired for a while now so do not have access to regs etc but a quick google came across this which seems like it has good information.
https://network-data-cabling.co.uk/wp-content/uploads/2024/11/Earthing-of-Metallic-Cabinets.pdf

You mentioned 3 phase in the first post, I would be very surprised if a rack was 3 phase as you would have a lethal voltage very close. If so it must have a warning sign.

The other think that comes to mind, is it for RJ45 patch panels or servers.

Regards,
Bob

[charliecoutas]

Hi Bob

Many thanks for yet more useful info. No, it's not comms gear, it's fairly hefty power supplies, some of them three phase in. I think my friend is doing all the right things but he can't find definitive answers to some questions.

The earthing cable being at least as thick as the incoming mains conductor makes sense, and the single point common earth point, your reference confirms that. I'll pass all the useful stuff from you and the others on to him.

Thanks again.
Best
Charlie

[RGV250]

Hi Charlie,
I am not so sure of the incoming cable, if you take that in a factory it is likely to be huge. I wonder if it is referring to the main supply to the rack?
Does the rack have a distribution board built in to it for the power supplies?

Bob

[charliecoutas]

Yes, a 32 amp 3 phase cable comes in (5 wires, 3ph, neutral and earth), to distribution box (supplied by customer).
The incoming cable has a 32 amp multipin plug on the end. The complete rack then goes to the customer and the only connection to the power is that cable. In the rack are two or three power supplies, I believe they are bi-directional - (where the power gets fed back into the grid).

Charlie

[RGV250]

Hi Charlie,
If you are feeding back into the grid I think earthing is the least of his problems, safety and isolation is going to be the bigger issue.
All this is way after I did regulations.

Bob

[charliecoutas]

Bob

I think most of the isolation and safety is taken care of in a wall-box that we can't fiddle with.

Charlie