Supply to UPS

[sjwkmje]

Hi All,

Asked to fit a supply to a large APC 4u rack mount UPS. It requires a 16A BS4343. I will be fitting a MK commando switched blue 16A surface mount socket next to the network rack. Its about 10 meters from a CU so cabling is fine in 2.5mm t+e with 4mm g/y to bond the cab from the earth bar inside the cu. Problem here is 17th edition, requirements for RCDs 522.6.7.

The cable will not be at a depth of 50mm due to it been a dry line partition wall. The unit is surface mount but the cable will go inside the wall and drop into the floor and back up to the cu the other side of the room.

I would not want to fit the UPS that will be feeding network servers and other equipment from an RCD supply. I would rather fit a c type 16a MCB.

Do I need to run the cable on the surface to avoid RCD projection or run it inside galv ducting inside the wall? Of course the galv ducting will have to be earthed along its entire length.

Any ideas?

Adam

 

[flameport]

Use armoured cable (SWA).

 

[Spark123]

Use SWA?

bah

 

[RF Lighting]

Yes, it must be surface mounted if using twin and earth or singles in non metallic containment.

If you want it to be flush then you must use either PVC cables in meatallic containment, SWA cable or pyro.

Are you aware of the requirements for high integrity earthing for supplies expected to have more than 10mA current in the protective conductor?

<edit> double pah!

 

[Pensdown]

I would not want to fit the UPS that will be feeding network servers and other equipment from an RCD supply.

So don't. As the designer there are five other options open to you - 522.6.6

Also, for loads of reasons, T&E and UPS supplies don't mix very well.

Edit....one of the reasons..

Are you aware of the requirements for high integrity earthing for supplies expected to have more than 10mA current in the protective conductor?

 

[sjwkmje]

Yes, it must be surface mounted if using twin and earth or singles in non metallic containment.

If you want it to be flush then you must use either PVC cables in meatallic containment, SWA cable or pyro.

Are you aware of the requirements for high integrity earthing for supplies expected to have more than 10mA current in the protective conductor?

<edit> double pah!

Actually t+e would be a bad choice as I will have to use high integrity earthing as RF pointed out. I over looked this. I was running a seperate 4mm g/y to bond the cab. Looking at the OSG section 7.6 for me to use high integrity earthing all that is required is for me to run a second g/y earth cable size > 1.5mm2 to the MK commando units earth terminal. If I use SWA then I can just 3 core and use the sheath as the additional earth.

SWA could be used here but the bend radius would be a bit tight but I can improve this.

Can I as I am not bothered with the full IP56 rating of the MK command unit drill a hole in the back of the unit fit a galv 20mm access, bolted into place with the SWA gland bolted to that then run the SWA right through into the unit. Would make a neater job as the SWA can be in the wall.

Problem with the SWA approach is the CU is plastic so I would need some way around this. Only way is a 20mm galv access box with crimps inside inside the floor with 2.5mm from this and an additional g/y.

Pyro would be a problem as my skills are a little rusty, and it has the same issues as SWA.

Adam

 

[electronicsuk]

Would the installation not be under the supervision of a competent individual? Also, why do you consider SWA into a plastic enclosure a problem?

 

[sjwkmje]

Would the installation not be under the supervision of a competent individual? Also, why do you consider SWA into a plastic enclosure a problem?

Never considered it good practice to bolt a gland into a plastic enclosure. From an earth point of view etc. I prefer to bolt into metal or use galv ducting if using MK master seal sockets outside.

It would be under the supervision of a competent individual.

Adam

 

[Taylortwocities]

Matthew must be reading my mind.
I guess this is a commercial premises?
If so an RCD may be omitted ".........where the use of equipment and work on the building fabric and electrical installation is controlled by skilled or instructed persons" (Quote from OSG)
More specifically see Reg 411.3.3

 

[electronicsuk]

Never considered it good practice to bolt a gland into a plastic enclosure. From an earth point of view etc. I prefer to bolt into metal or use galv ducting if using MK master seal sockets outside.

Fair enough, that's your prerogative. However, it's not at all uncommon to see SWA glanded onto plastic enclosures, nor does it present any safety issues if done properly. If you intend on using the armour as CPC then using a Piranhna earth nut on either end would still provide good CPC continuity even if the enclosure somehow became compromised.

It would be under the supervision of a competent individual.

In that case, as per TTC's post above, there is no specific requirement for the circuit to be RCD protected.

 

[sjwkmje]

Never considered it good practice to bolt a gland into a plastic enclosure. From an earth point of view etc. I prefer to bolt into metal or use galv ducting if using MK master seal sockets outside.

Fair enough, that's your prerogative. However, it's not at all uncommon to see SWA glanded onto plastic enclosures, nor does it present any safety issues if done properly. If you intend on using the armour as CPC then using a Piranhna earth nut on either end would still provide good CPC continuity even if the enclosure somehow became compromised.

It would be under the supervision of a competent individual.

In that case, as per TTC's post above, there is no specific requirement for the circuit to be RCD protected.

Thanks for all the advice, over looked the under the supervision of a competent individual. I will not add RCD protection. I will use 2.5MM SWA. This seems the best option.

As for plastic enclosures I would bolt in a gland with a earth nut and done it on many jobs I just don't like doing it. That's just IMHO. It was all ways something that was never allowed at be done work so I never did it until I started working for a new company where things were a little more relaxed.

Adam

 

[RF Lighting]

You need to use 3 core SWA and connect both the armour and the earth core at both ends into different screw terminals which are commoned together.

 

[electronicsuk]

You need to use 3 core SWA and connect both the armour and the earth core at both ends into different screw terminals which are commoned together.

That's what the regs call for, but has anyone here ever seen a BS4343 single outlet with multiple commoned earth terminals?

 

[RF Lighting]

Maybe it is not a suitable connector to be used on a circuit with high protective conductor currents then.

 

[electronicsuk]

Maybe it is not a suitable connector to be used on a circuit with high protective conductor currents then.

They're used in most datacenters, certainly all the ones I've been in, feeding PDUs powering anything up to 42 servers in a single rack. I'm not saying this is justification for using a connector that may not be fit for purpose, although perhaps there's something in the regs that does permit their use.