Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 11 replies
  • Subscribers 32 subscribers
  • Views 2479 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

LED Lighting Columns Inrush Current

Eddie Currents
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 ?   

  • 0
    Assuming a 10Amp Type B – this would be not suitable as this device can only withstand 50Amps

    B-type MCBs have an instantaneous trip threshold between 3x and 5x In - we usually use the upper number when we need to guarantee tripping (e.g. for ADS or fault protection) but in this case when you're wanting them not to trip it should be the lower (3x) figure to take into account. So 30A rather than 50A on that basis. (Ditto for your other numbers for other MCB types).


    Likewise with fuses, the BS 7671 charts only show the upper (slowest) limits of permitted operation - they may well operate more quickly in practice, so you'll likely need some manufacturer's data for the complete envelope.


    But as OMS said, there's also the duration of the starting current to consider - it's unlikely to be as large as they say for any significant duration and will likely decay quite quickly as capacitors etc start to charge up - so that will give you quite a bit of wiggle room, but still it's usually best to be starting with the right numbers.


    ....Or find a supplier of some better quality drivers that have more friendly characteristics.


       - Andy.

Community Rules & Guidelines