Earthing comms cabinets

[stu1979uk]

Hi guys,

Yesterday I was asked to price a job for earthing comms cabinets, the room has 4 rows of 6 cabinets with a D.B. in a cupboard within the room.

I was thinking of installing 4x 10mm earth cables, 1 for each row,with bonds across the cabinets within the rows, would this be sufficient?
I was also wondering if it would be ok to connect directly into the D.B's earth bar or would it be better practice to install an earth bar at the D.B. and take a single earth back to the main switch rooms earth bar?

The only earth currently present is through mains supply of the equipment ie the plugs on the servers which are screwed into the cabinet.

Any thoughts/advice welcome, thanks

Stuart

 

[SimpleSimon]

Are these cabinets fixed equipment?

 

[stu1979uk]

they are all floor standing units I don't think they have wheels

 

[grantr37]

there should really be a "clean earth" separate and isolated from the other earths for this kind of equipment (some might have a functional earth) but I dont think that the customer will want to pay for adding a new incoming earth, large copper bar etc, you will need to ask them what they want, and if they want this then it would be better...otherwise it will need to be 4 chains of 10mm going back to a 10mm copper earth terminal block at the DB..

10mm Earth is absolutely ideal, make sure you get some serated/jaggy washers and use some sandpaper to rub a little bit around where you drill for the the earth connection points to give a good contact, as you will need to rub off the enamel/paint...

 

[stu1979uk]

I thought something along those lines would be required. The only problem is there is no earth bar just now, only the one inside the D.B. so I would have to install the bar and run a cable back to the main earth.
Assuming I do so would this be considered a clean earth?
Its all going to depending on cost which is why I can see them wanting to connect to the earth bar in the D.B.If that is the case could that cause problems?

cheers for the help grant

 

[bugsy]

all proper comms rooms should contain clean earth bar with a disconnecting link, this should be connected to the main earth bar and installation incommer clean earth is normally defined with a cream sheath

 

[stu1979uk]

So to do this 100% correctly there should be;
a cream sheathed earth coming from the main earth bar at source to a new clean earth bar fitted in the comms room. From there a 10mm earth to each of the 4 rows of racks.

Alternatively the cheap way would be to run 4 10mm's from the racks back to the earth bar inside the D.B. in the comms room but this could potentially introducing a high earth current onto that D.B.

 

[bugsy]

that;s about it , but the clean earth bar needs a disconnecting link

 

[BlueToBits]

My personal experience is with sound equipment which may be different, so I stand by to be corrected....
If the earth to the rack is to be the central earthing point for the functional earth to all equipment across the network, I would agree with the above. If the earth to the rack is for safety purposes and if the system requires a clean functional earth, the cases of all interconnected equipment across the network should be isolated from the functional earth and earthed locally through the equipment's power socket. The functional earth is then connected to the clean earth. If the functional earth and the safety earth are connected together at any point in the system it would 'contaminate' the clean earth, worse still, the clean earth may become a parallel path to the safety earth for other circuits in the building and could cause considerable damage or danger in the event of a fault elsewhere. Specifications for 'clean' earths vary. Sometimes a clean earth is required to be connected directly to the CMET, other times it is derived from buried rods and tapes. If earthing the rack is for electrical safety purposes only, then a 4mm tail between the rack and the earth at the power supply (fused connection unit say) may be all that is required. It would depend on how you were supplying and distributing power to the rack. It does no harm to do it in 10mm but your client won't thank you for charging him for unnecessary work and materials.

 

[bugsy]

Just to clarify are you talking about functional earth or protective earth

 

[Engineer54]

My personal experience is with sound equipment which may be different, so I stand by to be corrected....
If the earth to the rack is to be the central earthing point for the functional earth to all equipment across the network, I would agree with the above. If the earth to the rack is for safety purposes and if the system requires a clean functional earth, the cases of all interconnected equipment across the network should be isolated from the functional earth and earthed locally through the equipment's power socket. The functional earth is then connected to the clean earth. If the functional earth and the safety earth are connected together at any point in the system it would 'contaminate' the clean earth, worse still, the clean earth may become a parallel path to the safety earth for other circuits in the building and could cause considerable damage or danger in the event of a fault elsewhere. Specifications for 'clean' earths vary. Sometimes a clean earth is required to be connected directly to the CMET, other times it is derived from buried rods and tapes. If earthing the rack is for electrical safety purposes only, then a 4mm tail between the rack and the earth at the power supply (fused connection unit say) may be all that is required. It would depend on how you were supplying and distributing power to the rack. It does no harm to do it in 10mm but your client won't thank you for charging him for unnecessary work and materials.

As BlueToBits has just described, is spot on!! As he states a True ''Clean Earth'' can only be derived from a dedicated earth rod ground field. For smaller equipment requirements, a clean earth conductor is taken back to the CMET...
In both cases, the clean earth must remain totally isolated from any building earthing, ....they do not perform the same function as each other...

 

[stu1979uk]

Its for protective purposes so not a functional earth. I didn't consider the fact that the equipments earth would contaminate a clean earth, good point.

Currently the racks are connected to the ring circuit within the room, therefor the racks are earthed via the equipments metal casing being screwed into the rack.

So taking into account the earth is for protection I could install 4mm (with mechanical protection) or 10mm (without mechanical protection) onto the racks and terminate at the D.B. within the room. This would also be the same earth which is currently supplying the equipment within the racks via the ring circuit.
This would give a high integrity earth sufficient for a protective conductor current likely to exceed 10 mA.

that sound about right?

 

[BlueToBits]

On the face of it, I would say that would be more than adequate.
We sometimes fit an industrial 16A power inlet to the rack and install whatever distribution is needed from that. An earth tail from the inlet is taken to the rack via a nut and bolt. The rack is then earthed through the power cable.