Correct Contactor and MCB

[richard1barker@]

Hi All
I am going to be installing some high bay lighting which will consist of 3 x 400w metalhalide fittings so 1200w in total with runs of approx 60 meters in 2.5mm cable. I will be wiring the circuit via a 20amp contactor with a control circuit of a two way switch system to activate or pull in the contactor coil when swicthed on. Could anyone advise me on the size and type of RCBO to use for the lighting circuit and if when the switch is used to turn the lights on if it will trip one or both RCBOs in use.(One RCBO for light circuit and one for switching circuit)

 

[Brightspark2]

Thanks again for response Darkwood as I've wired it in 2.5mm this won't be a problem which is why I put a bigger size than normal even though I should have checked first to be sure. Also have u ever heard of two RCBOs tripping each other when used in this way on a contactor if on same or different phases?

If they are on separate circuits then a fault on one circuit will only operate the RCBO protecting that circuit.

 

[Darkwood]

I understand now why so many get it wrong.... 10(c) or 16(c) cannot guarantee it will prevent nuisance tripping as stated in my earlier post and using the trip thresholds of the mcb and the inrush effect of 3 400w metal halides together which is not the same as calculating it for say fluorescents then you come up with the answer 16(d) (will cover 3 or 4 lamps) or 20(c) (will cover 3 lamps).

When calculating different types and no's of inductive lamps there is no rule of thumb you can use you either have to seek guidance from tech' advice or tables or work it out the long way which again you have to seek info on predicted inrush currents and trip thresholds.

This needs to be done before designing the rest of the circuit and calculating cable size etc.

Same with motors, transformers ......all variable inductive loads dependant on size type start-up characteristics .....

 

[richard1barker@]

Hi I'm registered with NAPIT and their technical help line operaters always advise using rcd protection on all circuits weather domestic or commercialwhen not mechanicaly but if I can get away with mcbs for light feeds will use them if this complies with regs on a commercial\industrial installation

 

[Brightspark2]

I understand now why so many get it wrong.... 10(c) or 16(c) cannot guarantee it will prevent nuisance tripping as stated in my earlier post and using the trip thresholds of the mcb and the inrush effect of 3 400w metal halides together which is not the same as calculating it for say fluorescents then you come up with the answer 16(d) (will cover 3 or 4 lamps) or 20(c) (will cover 3 lamps).

When calculating different types and no's of inductive lamps there is no rule of thumb you can use you either have to seek guidance from tech' advice or tables or work it out the long way which again you have to seek info on predicted inrush currents and trip thresholds.

This needs to be done before designing the rest of the circuit and calculating cable size etc.

Same with motors, transformers ......all variable inductive loads dependant on size type start-up characteristics .....

All very true. Although the figures come out quite excessive from experience. 20A type C!


What type/rating of MCB would you calculate for 6x250W fittings?. Using your square D figures.

 

[MDJ]

I'd be considering C rated MCBs, and would fight to the death to avoid RCBOs myself on a commercial lighting system.

 

[MDJ]

Hi I'm registered with NAPIT and their technical help line operaters always advise using rcd protection on all circuits weather domestic or commercialwhen not mechanicaly but if I can get away with mcbs for light feeds will use them if this complies with regs on a commercial\industrial installation

rollocks do you mean Crapit? how the hell can they suggest RCDS OR RCBOs on commercial lighting LOL

 

[Darkwood]

All very true. Although the figures come out quite excessive from experience. 20A type C!


What type/rating of MCB would you calculate for 6x250W fittings?. Using your square D figures.

6 x 250w metal halide are good for 20(c) or 16(d) coincidently ... what you have to remember here is the nature of the trip threshold of an mcb as it isnt fixed ie a type C will trip 5-10 times rated current if you have a load that 7 times inrush then you have to remember the mcb can trip at 5 times but in reality you could get away with it rarely happening but thats ok on small jobs but big jobs where losing lighting can be a big issue like large complexes, factories etc even the one off events can create alot of problems ..... i state the mcb ratings i put forward guarantee no nuisance tripping due to inrush, it is the case a lower rated may do the job fine with little or no incidence that you know of but the fact is there is always the probability it will due to the design.

 

[Guest55]

21 posts and no-ones mentioned the exemption for rcd's needed by cables buried in walls due to commercial installations having a duty holder / supervising person obligation.

its all there in black & white
;-)

 

[Brightspark2]

6 x 250w metal halide are good for 20(c) or 16(d) coincidently ... what you have to remember here is the nature of the trip threshold of an mcb as it isnt fixed ie a type C will trip 5-10 times rated current if you have a load that 7 times inrush then you have to remember the mcb can trip at 5 times but in reality you could get away with it rarely happening but thats ok on small jobs but big jobs where losing lighting can be a big issue like large complexes, factories etc even the one off events can create alot of problems ..... i state the mcb ratings i put forward guarantee no nuisance tripping due to inrush, it is the case a lower rated may do the job fine with little or no incidence that you know of but the fact is there is always the probability it will due to the design.

Very true squire :icon4:. You speak sense.










Which is rare for this site ha ha

 

[richard1barker@]

What does this exactley mean and does this make it as safe to not use rcd becuase of it, what about outside lts and sockets on a commercial installation the rcd rule must still apply to them. Also may be a good idea to use rcbos on lighting anyway with length of runs which may exceed ZS with mcbs of type c and d nature

 

[MDJ]

What does this exactley mean and does this make it as safe to not use rcd becuase of it, what about outside lts and sockets on a commercial installation the rcd rule must still apply to them. Also may be a good idea to use rcbos on lighting anyway with length of runs which may exceed ZS with mcbs of type c and d nature

you design the circuit so it won't exceed maximum ZS readings Richard??

 

[iamelectric]

i was always taught at college to use the 1.8 for inductive lamps , and we used to fit a few hundred low bays every year in my work and never used a D type , the only time we have ever used a D type was for a supply for a welder , the 3 fittings only add up to 1.2kw 5.3 amps a type c mcb trips between 5 and 10 times full current load , also when you come to test the circuit the max zs for a type D is far lower so may not achieve disconnection times

 

[Brightspark2]

What does this exactley mean and does this make it as safe to not use rcd becuase of it, what about outside lts and sockets on a commercial installation the rcd rule must still apply to them. Also may be a good idea to use rcbos on lighting anyway with length of runs which may exceed ZS with mcbs of type c and d nature

Sockets yes but more than likely not lights. Remember why we use a 30mA RCD its there to protect persons. Which isnt needed for exterior lighting.

As MDJ said you design the circuit not to exceed the maximum ratings.

 

[richard1barker@]

There is only one route we can wire the lights in question the length is due to the size of the unit and its height and these are the lights the client wants to use hence why we have kept only three to a circuit so the only way to ensure compliance with regs with these mcbs is to use rcbos MJD??

 

[iamelectric]

why would you not use rcbos on commercial lighting MDJ? we use rcbos all the time for commercial lighting