Two single phases through one isolator?

[schooleydoo]

Morning guys,

I'm new to this so bear with me!

I have two 16a single phase machines that I want to supply with a single cable. I want to use one neutral and earth for two separate circuits. Is this possible? I seem to remember when N is a load carrying conductor, combining two loads could burn it out?

Also, can I use a three phase isolator switch (like this) and just leave L3 empty?

Like this:

 

[telectrix]

bad design there, mate. as it's drawn, your N cable has to carry the load of both machines.

 

[schooleydoo]

Yeah thats what I was wondering too, with my load carrying conductor comment. Ah well, thank you for clarifying

Assuming no problems if I took two Neutral lines back to the consumer unit? Any ideas on the switch?

 

[telectrix]

you could feed to the isolator in 6mm, then split to the 2 ocpd's in 2.5mm, 1 for each machine. however, i would fit separate isolaters, each 1 fed from DB, with the ocpd's in the DB, not after the isolator. the feed has to be protected at the DB with MCB/RCBO, so why fit additional ocpd's?

 

[schooleydoo]

I think I may be overcomplicating things!

We are installing machines at a industrial site. The customer wants to be able to switch both machines on from a single point, at the machine itself. So I want to put both machines circuits through one isolator switch and mount on the side of the machine.

I wasn't sure about the customers protection on the DB, so thought it best to protect our machines ourselves.

The customers supply will be three phase. So I thought I could use L1 for machine A, L2 for machine B and terminate L3 at my isolator switch and go no further with it. I would then run two Neutral lines back to the DB, one for each machine. Would this work?

The only problem I can foresee is A - running two neutral cables through one isolator switch and B - their 3 phase supply will only have one Neutral. If the cable dia was correct, could I use one N for both machines load?

 

[telectrix]

each m/c is 16A, so from a 40A MCB, 6mm to isolator. L1 for m/c 1 L, L2 for m/c 2 L, L3 for N. then cable each m/c in 2.5mm, fitting appropriate size OCPD (16A)for each.

 

[telectrix]

don't forget, the isolator needs to be rated for both loads added. say a 40A.

 

[Knobhead]

Using two phases at 16A, one per machine you will have 16A in the neutral. So loading of the neutral shouldn’t be a problem. But I will say I don’t like the set up.

 

[telectrix]

i'd prefer 2 isolators with independent feeds from DB. OCPDs at DB.

 

[D Skelton]

Havent really had time to think about circuit arrangement but can anyone see a use for contactors in this scenario? Maybe arrange two seperate circuits from DB through contactors?

Not really my area of expertise so I can't say for sure.

 

[schooleydoo]

Got it, massive thanks everyone

Also just found out machine 1 is actually 13a, machine 2, 12a. So downgraded to 32a protection.
When you say L3 for N, this is from my switch onwards, correct? I can't use the L3 line all the way back to the DB can I?! This will not be a balanced load.

Switch - Buy Non-Fused Switch Disconnectors 3P open type switch disconnector,32A Ie Telemecanique VBD1 online from RS for next day delivery.

MCBs - Buy MCBs 16A MCB 2 Pole Type B ABB S 202-B 16 online from RS for next day delivery.

The earth will come straight into our PE terminal inside the cabinet from the supply line, it will not go through the isolator. Is this OK? Technically in the event of an earth fault, you probably need to be able to isolate the machine? But if there is an RCD on the DB, it would protect it anyway...

 

[eletricaz]

Why do the machines have to be on different phases why not put them both on one phase protected at 32a and use a single phase isolater and double up on the outgoing side of the live and neutral side this also eliminates the chance of having 400v between the 2 machines if they are close together and the person using them safer in the event of a fault

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[eletricaz]

Forgot to mention make sure you add your own protection for each switch after the isolator 16a should be fine as they are machines and may have motors on them

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[snowhead]

4 pole Isolator
L1, L2 on 2 poles
L3 secured in a terminal block within the box.

Loop the neutral across the other 2 poles on the incoming side.

The isolator only needs to be rated at the highest of the 2 loads, but I'd use a 20amp as a minimum.

Then whatever protection you want for each machine after the isolator

How far away from the master isolator is the 2nd machine, you may still need a local isolator for that, or is it built in?

The earth will come straight into our PE terminal inside the cabinet from the supply line, it will not go through the isolator. Is this OK? Technically in the event of an earth fault, you probably need to be able to isolate the machine? But if there is an RCD on the DB, it would protect it anyway...

You can't run the earth separate, it needs to be with the phases, if it travels a separate route it may get accidentally disconnected.
I can't quote a reg for this but there must be one.

 

[schooleydoo]

Electraz, that would be ideal but the customer doesn't want to change their existing power drop to the install area or DB.

Snowhead... 4 pole as in 3P+N? As far is I can tell, there is no earth on these? So I assumed it just bypassed the isolator but like you, would be surprised if it wasn't supposed to be switched.

So with your method I would be using L1 and L2. L3 would not run through the switch but safely terminate inside the cabinet and remain unused. N would split after the switch as per my drawing above. I don't understand what you mean by looping N between two poles on the incoming side though? A jumper cable Between L3 and N? Why? Just to make splitting N tidier? Out of L3 and N on the switch output.

The two machines are actually inside one enclosure, about 500mm apart. This is machine A's enclosure. You can see the main isolator on the side, running up to the MCB and onwards. Machine A is a UV dryer. Machine B is a plasma generator.