Calculating inrush current for magnetic ballasts

[davesparks]

I need to, as accurately as possible, establish what the inrush current is for some large metal halide floodlights with magnetic ballasts and I'm drawing a blank.

This is for a switchgear/DB replacement I am in the planning stages of and need to know whether these circuits can be fed from MCBs or if I need to use MCCBs or if fuses would be OK.
These are currently fed from 63A MCCBs, but it would be advantageous if they could be run on MCB's instead.

There are 4 TP circuits each feeding a floodlight column. Each column has 9x 2kW MBIL type discharge lamps on it with magnetic ballasts connected in delta with loads evenly distributed (3 connected between L1 and L2, 3 between L2 and L3 and 3 between L1 and L3).

I've tried asking schneider as they can usually look up a maximum number of fittings for a given MCB, but they dont have info for this size fitting and can't help without the inrush data.

So I've asked the ballast manufacturer, Thorn, for the inrush data and they apparently don't know. But they did come up with a max starting current (not inrush) of 9.3A at 415V.

Can anybody help with calculating the inrush for these?

 

[freddo]

These floodlights are around 25 years old.

I do like these lights, the 220,000 lumen output is certainly impressive.

 

[davesparks]

I do like these lights, the 220,000 lumen output is certainly impressive.

They're pretty damn bright, even in daylight you can see the effect on the pitch.

 

[marconi]

See first table in which makes recommendations on ratings and types of CB and contactors:

https://www.veelite.com/wp-content/uploads/Protection-of-Lamp-Circuits3.pdf

I will see if I can find anything else.
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See page 4 of:

http://www.moeller.net/binary/ver_techpapers/ver955en.pdf

and see table in which has starting surge current and its typical duration:

Discharge lamps - Electrical Installation Guide - https://www.electrical-installation.org/enwiki/Discharge_lamps

 

[Lucien Nunes]

I too would test on site, but using a scope and a shunt or CT. There's nothing like seeing a picture of it to get a proper feel for what is going on.
There are three separate causes of inrush here:
PFC capacitor charging: less than half a cycle, instantaneous current could approach PSCC for a time constant of Zs x C. Worst when contacts close just before V=Vpk
Magnetic saturation: Worst when contacts close at V=0 since this causes the greatest magnetic excursion. Instantaneous current could approach V/(Zs+Rwindings). Duration usually a few cycles while the core gets into step with the mains.
Starting current: Lower, longer, depends on lamp and well documented by the lamp makers.

Watching the waveform would reveal each of these components and allow a good estimation of the I²t that the breaker has to put up with.

 

[Ian1981]

I too would test on site, but using a scope and a shunt or CT. There's nothing like seeing a picture of it to get a proper feel for what is going on.
There are three separate causes of inrush here:
PFC capacitor charging: less than half a cycle, instantaneous current could approach PSCC for a time constant of Zs x C. Worst when contacts close just before V=Vpk
Magnetic saturation: Worst when contacts close at V=0 since this causes the greatest magnetic excursion. Instantaneous current could approach V/(Zs+Rwindings). Duration usually a few cycles while the core gets into step with the mains.
Starting current: Lower, longer, depends on lamp and well documented by the lamp makers.

Watching the waveform would reveal each of these components and allow a good estimation of the I²t that the breaker has to put up with.

Took the words right out of my mouth ?

 

[marconi]

Davesparks: I am sure you have thought of it but just in case you have not, maybe you could include some delay-on timers to stagger the energisation of the lamps? Take care to select different delay on times so that there is no overlap of switch-ons and thereby reduce the switch on surge current. You get the idea.

80.11.0.240.0000 | Finder SPDT Timer Relay - 0.1 → 20 s, 0.1 → 20 min, 0.1 → 24 h, 1 Contacts, ON Delay, DIN Rail | RS Components - https://uk.rs-online.com/web/p/timer-relays/0221086?cm_mmc=UK-PLA-DS3A-_-google-_-CSS_UK_EN_Automation_%26_Control_Gear_Whoop-_-Timer+Relays_Whoop-_-221086&matchtype=&aud-830986524389la-298837490304&gclid=CjwKCAjwm_P5BRAhEiwAwRzSOw5_BDyueNDFrKi6FJUJv880ONKoE8rhoe1hSfRa0ltAW703ehRH_BoCPhQQAvD_BwE&gclsrc=aw.ds

 

[pc1966]

Do you know the trip characteristics of the current MCCB? If the "instant" trip point is comparable to a 63A C or D curve MCB you should not have any issues on the tripping. I guess whatever contactor(s) are switching them have survived enough time to be proven good enough!

 

[davesparks]

Davesparks: I am sure you have thought of it but just in case you have not, maybe you could include some delay-on timers to stagger the energisation of the lamps?

I had thought about that, but the cost of doing that would negate the financial benefit of installing MCB's as opposed to MCCBs.

I could fit MCCBs which match the tripping spec of the existing protection, but the aim of this exercise is to reduce installation costs for a local good cause who have a shoestring budget.
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Do you know the trip characteristics of the current MCCB? If the "instant" trip point is comparable to a 63A C or D curve MCB you should not have any issues on the tripping. I guess whatever contactor(s) are switching them have survived enough time to be proven good enough!

I have looked up the information and tried to assess it that way, but I think I may hav emade an error in my maths so will be looking at it again later.
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I too would test on site, but using a scope and a shunt or CT. There's nothing like seeing a picture of it to get a proper feel for what is going on.
There are three separate causes of inrush here:
PFC capacitor charging: less than half a cycle, instantaneous current could approach PSCC for a time constant of Zs x C. Worst when contacts close just before V=Vpk
Magnetic saturation: Worst when contacts close at V=0 since this causes the greatest magnetic excursion. Instantaneous current could approach V/(Zs+Rwindings). Duration usually a few cycles while the core gets into step with the mains.
Starting current: Lower, longer, depends on lamp and well documented by the lamp makers.

Watching the waveform would reveal each of these components and allow a good estimation of the I²t that the breaker has to put up with.

I should be able to rustle up a scope, it's the CT I may struggle with.

I can't see a way of switching them on at the right time to catch the maximum possible inrush though.

A nice scope trace that I can send to schneider and let them take it from their would be perfect.

 

[pc1966]

I have looked up the information and tried to assess it that way, but I think I may have made an error in my maths so will be looking at it again later.

If you have the model and any settings then folk here might be able to check to help out.

 

[davesparks]

I think @pc1966 might have solved it, at least solved it enough for what I need.

The existing MCCBs are Dorman Smith Load line AA 63A set at 63A. If I am reading the data sheet correctly they have an instantaneous trip of 400A.
So logically anything with an instantaneous trip equal to or above 400A should be fine?

But then I have hardly slept for a week so I have probably missed something.

 

[Lucien Nunes]

I can't see a way of switching them on at the right time to catch the maximum possible inrush though.

I would take a series of measurements, switching each lamp or circuit on in turn. Put the voltage on one trace (using a PT if needed) and the current on another, and you can see what happened each time and make up an average.

 

[pc1966]

The existing MCCBs are Dorman Smith Load line AA 63A set at 63A. If I am reading the data sheet correctly they have an instantaneous trip of 400A.
So logically anything with an instantaneous trip equal to or above 400A should be fine?

That is my thought. While there might be some strange current-time aspect that trips on the thermal part, usually that is for a second or more of time which is longer than any reasonable switching-on surge for L or C.

400A & 63A is a bit more than low-threshold C-curve MCB but less than a D-curve.

You might not even need 63A, as a 50A, or possibly even 40A D MCB would meet that 400A trip point and would be 27kW resistive TP @ 230V so probably OK for 9kW of discharge lights.