One For the leckies (industrial/3phase)

[barracuda816]

Afternoon guys,

Im happy with most aspects of leckieing and am reasonably well versed in the 17th ed but i have only experiance in domestic. I have recently started working for a company that builds 3phase (induction motors) machinery. We have the standard 3 phase supplies (16, 32 and 63) in the building.

Some equipment we buy in has 32a plugs wired with 5core 2.5mm2 the design current for the machine is 10a per phase (so would be fine on a 16 D Type, without getting into Zs values ect.) But im not happy putting 2.5mm on anything where the protective device is 32a. The company also wire 63amp pluged machines with 6mm2 (the current draw of that machine is aprox 45a) but again to me 63a mcb, plug is not fused so the wire from the plug to the machine should be able to handle the max current of the protective device (MCB).

Thats along way of saying i would wire the Following MCBs

16a = 2.5mm
32a = 4mm
63a = 10mm

Am i right? i couldnt find anything that covers this as its not for the fixed installation. Should i just be looking at the spec sheet for the flex (im used to dealing mostly with T+E on single phase so can just ref the tables in the OSG or 17th)? Is there anything laid down anywhere for this?

Thanks in advance guys and apoligies if this is a Daft question.

 

[brightspark]

u spur of a ring main with 2.5mm on a 32 amp breaker probably short leads from plug to machine

 

[barracuda816]

true but on a spur unless im mistaken a max theroretical draw of 26a (2x 13a) anything more would be protected by an fcu.

 

[brightspark]

true but on a spur unless im mistaken a max theroretical draw of 26a (2x 13a) anything more would be protected by an fcu.

u just pointed out there using 2.5mm on 30 amp sockets and the load is only 10 amp

 

[carbon]

Do the machines not have any thermal overload fitted in case of a motor fault. If they do would that not protect the incoming cable?

 

[WorkshopChris]

The company also wire 63amp pluged machines with 6mm2 (the current draw of that machine is aprox 45a) but again to me 63a mcb, plug is not fused so the wire from the plug to the machine should be able to handle the max current of the protective device (MCB).

From real world experience this is standard practice 6mm or 10mm, provided the machine has its own over current protection.

 

[Robotstar5]

The cable on the machine is normally only sized to accommodate the max design current draw so may be smaller than expected for that size plug

 

[barracuda816]

u just pointed out there using 2.5mm on 30 amp sockets and the load is only 10 amp

The Non fault current is aprox 10a. There is a Thermal overload on the machine but if the fault is on the control side or before the machine then the only protection is the MCB

 

[barracuda816]

The cable on the machine is normally only sized to accommodate the max design current draw so may be smaller than expected for that size plug

This is what i see (im sure as a cost saving exersize) but im concerned about the fault protection afforded to a 2.5mm by a type D 32a MCB

 

[WorkshopChris]

There is a Thermal overload on the machine but if the fault is on the control side or before the machine then the only protection is the MCB

Have i read this wrong or are you saying the only protection for the control side is the 63a mcd in the building, not on the machine.

 

[Robotstar5]

This is what i see (im sure as a cost saving exersize) but im concerned about the fault protection afforded to a 2.5mm by a type D 32a MCB

The MCB only protects the socket, the machine could have 0.75mm cable on a 32A plug if the machine only drew a couple of amps.

 

[MattH]

That flex would usually run straight into a DOL with overload protection so I can't see how the 2.5mm would ever see more current that the DOL is set at.
If you are suggesting that in the event of the DOL's thermal overload faulting and allowing more current to be drawn then that would require a secondary fault after the DOL.

If you trust a correctly sized MCB then why not trust a thermal overload?

 

[dobbslc]

Multicore flex usually has a high rating than T&E, worth having a look in the regs or Batt cables or Cleveland cables websites.

Also the 32amp MCB in the dist board is only there to protect the circuit cable not anything plugged into it.
So if you had a fault in the machine or the circuit wiring itself that draws a massive current, the MCB should trip before the circuit cable is damaged.

 

[barracuda816]

Have i read this wrong or are you saying the only protection for the control side is the 63a mcd in the building, not on the machine.

Kind of, generally there is, from the plug/ MCB there is roughly 20mts of cable wired to the control pannel where a transformer tapps of one of the phases whislt all the phases run to the contactor then to the OL the tapping for the transformer runs to a fuse. Then to the limit switches ect.

The MCB will protect the socket, plug and fixed installation. the OL will protect the wiring after the OL as will the fuse after the transformer. Now in my mind you have 20mts of cable (plus the little bit in the control box before the OL and Fuse) that is not Protected. If the insulation is damaged on that cable it could draw enough to melt/set fire to that cable before the MCB trips.

Is it likely, no. But i would have thought there would be some regs somewhere covering this senario?

 

[brightspark]

why don't u take it up with the manufacturers or the boss if your concerned about it and see what they say

 

[brightspark]

join here on this site if u want anything clearing up. it doesn't get any better the advice givenhttp://www.voltimum.co.uk/

 

[barracuda816]

I have mentioned it to the boss but they are not up on electrics/ electrically qualified and are happy if the manufacture is.

Thanks Brightspark, i will take a look

 

[WorkshopChris]

If the insulation is damaged on that cable it could draw enough to melt/set fire to that cable before the MCB trips.

Personally i like to see a MCB or Fuses to cover the hole machine directly after the isolator and maybe a second for the control side depending on the first mcb size. In my world its all about limiting damage and getting kit back up and running asap. So everything is specified around this.

The others may have some input on this but generally if a cable or insulation is damaged then it will either short and almost instantly trip the supply or arc itself out to clear the short.(never seen a flex melt or catch fire) Depending on the supply size and breaker ratting.
However if the supply to a machine was on a 20m flex i would be looking at protecting the circuit with a RCBO or RCD.