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LED Lighting Columns Inrush Current

I am looking at installing around 21 LED Lighting Columns to a Car Park. It states on the product data sheets that the Lamp Heads are 890mA / 68watts each (I think the 890mA refers to the output current of the driver and not the input current which will be nominally P/V excluding the efficiency of the driver). I have checked all the volt drops and cable calcs which have calculated out at 6mm 3core SWA buried underground. I am splitting the Lighting columns up into circuits and I would prefer to have 7 columns on each circuit. From the information provided on the data sheet it states that the inrush current is 57Amps per column which does seem a little high considering such a small load. This would mean when I select a suitable Overcurrent Device from BS7671 Time Current Curve Charts to withstand the 57Amps x 7 Columns = 399Amps inrush current. Full Load Current being 68Watts x 7 = 476Watts equates to around 2Amps. As 57Amps inrush is quoted (sum of + 399Amps) for the one Lighting Column then the following is concluded:

Assuming a 10Amp Type B – this would be not suitable as this device can only withstand 50Amps up to 5 Seconds so it will cause an instantaneous trip (this would normally be the device i would choose as default)
Assuming a 10Amp Type C – Suitable for a max of 1 Lighting Column as this device can only withstand 100Amps up to 5 Seconds so it will cause an instantaneous trip with 2 Lighting Columns wired on the one circuit
Assuming a 10Amp Type D - Suitable but only for a max of 3 Lighting Columns as this device can only withstand 200Amps up to 0.4 Seconds and withstand up to 100Amps over 5 Seconds so it will instantaneously trip with more than 3 Lighting Columns wired on the one circuit (i would usually use Type D only on motor circuits and not the usual device of choice for protecting Lighting Circuits)

Deduced from the data sheet and if all is correct, we could only wire a maximum of 3no Lighting Columns per circuit but this using a Type D Overcurrent Device to be sure no tripping will occur during initial spike. This has confused me slightly because I have never seen such high inrush currents like this for such small LED Lighting Loads.

I wondered if any Electrical Designers/Electricians had queried this before with manufacturers and also whether i am missing something. One would expect you could have 7 Columns per circuit as such a small load of 2Amps but the inrush would surely take out the protective device ?

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0 Former Community Member over 4 years ago

Probably worth pointing out that the inrush current, whilst reasonably large, is also very short lived - the manufacturers data probably says it's something like 57A but that is only likely to exist for about 150 microseconds - ie about 0.00015 seconds.

This is so short as to make time current curves almost useless, you need another approach such as:

Wire the circuit to your proposed column groups (7No) in say 6.00mm2 and protect with a 63A fuse (you aren't worried about overload protection at this point - only short circuit and inrush protection)

Put a 10A fuse in the cut out assembly to manage the inrush of the single lamp and to act in concert with the other columns for overload protection of the circuit (you are assuming here that you won't get the same overload fault on every column simultaneously)

You could run a 3 phase circuit around every column to 3 phase cut outs with the luminaire connected on the relevant phase to the cut out (or as Alan suggested, loop a single phase circuit and use 3 circuits)

In practice, the very short duration of inrush at 0.001 seconds as a very conservative upper bound, won't ever trouble a MCB which usually has a minimum time to operate of around 0.01 seconds (ie 10 times longer) - consequently you are very unlikely to delatch an MCB - but fuse technology in this application is preferable

Regards

OMS
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0 Former Community Member over 4 years ago

Probably worth pointing out that the inrush current, whilst reasonably large, is also very short lived - the manufacturers data probably says it's something like 57A but that is only likely to exist for about 150 microseconds - ie about 0.00015 seconds.

This is so short as to make time current curves almost useless, you need another approach such as:

Wire the circuit to your proposed column groups (7No) in say 6.00mm2 and protect with a 63A fuse (you aren't worried about overload protection at this point - only short circuit and inrush protection)

Put a 10A fuse in the cut out assembly to manage the inrush of the single lamp and to act in concert with the other columns for overload protection of the circuit (you are assuming here that you won't get the same overload fault on every column simultaneously)

You could run a 3 phase circuit around every column to 3 phase cut outs with the luminaire connected on the relevant phase to the cut out (or as Alan suggested, loop a single phase circuit and use 3 circuits)

In practice, the very short duration of inrush at 0.001 seconds as a very conservative upper bound, won't ever trouble a MCB which usually has a minimum time to operate of around 0.01 seconds (ie 10 times longer) - consequently you are very unlikely to delatch an MCB - but fuse technology in this application is preferable

Regards

OMS
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LED Lighting Columns Inrush Current

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