Author Topic: Probe start hot restrike  (Read 1214 times)
sol
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Probe start hot restrike « on: November 27, 2018, 07:13:22 AM » Author: sol
So I have been wondering about this for years. When I started in high school, in 1992, the new gymnasium was only built about 4 years prior. It was lit with 16x400 MH, probe start M59 lamps. The ballasts were remote mounted in a ventilation room. Everything worked quite well.

Inevitably, as in all HID systems, the question of hot restrikes was considered by teachers, administrators and students. Whenever it happened, the instructions, which were followed mostly to the letter, were that the switches were to be turned off for 10-15 minutes and then turned back on. I have always wondered why that was the practice. The huge 1kW MH lamps at car dealers, for example, wouldn't be switched off for a cooling off period after brief power interruptions. Also, the gym having probe start lamps, no ignitor would go bad if the switches were left on.

When I got my job there (6-7 years later), no one talked about it. Hot restrikes were mostly (I think) without switching off the lights. If a presentation needed a short dark period (to show a 2-3 minutes video, for example), I would appoint myself as the lighting switcher and I would immediately (before the event started) turn off half the lights. That way, when the 2-3 minutes of required darkness were over, I would have half the lights cooled down and ready to strike, but I would turn on all switches and the ones that were lit before the video would cool down more until the bimetallic strip closed the starting probe.

Unfortunately, all this is now irrelevant as there are 6xF32T8 fixtures instead of the MH.
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dor123
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Re: Probe start hot restrike « Reply #1 on: November 27, 2018, 08:34:06 AM » Author: dor123
There are no critical reasons for turning off the lights during cooling down of probe-start MH lamps. When the lamp is hot, the aux probe is shorted to the main electrode by a bi-metal to switch it off, so no electrolytic corrosion will happen in the area of the starting electrode, so the aux discharge can't be formed before this bi-metal opens. Theoretically the bi-metal should open when the arctube is far colder than the restriking temperature, so that the main discharge will strike directly as soon as the bi-metal opens without the appearance of the aux discharge like in the case of mercury lamps, but as I've heard and seen, that in most probe-start MH lamps, the bi-metal opens when the arctube is still hot to be restruck, so the aux discharge appears similar to mercury lamps.
But when the aux discharge appears in the arctube, the CWA ballast supply its max OCV (Usually 380V) than when the lamp is operating, so this may stress the transformer windings and overheat them. This is true also with mercury lamps on CWA ballasts.
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Medved
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Re: Probe start hot restrike « Reply #2 on: November 27, 2018, 12:02:14 PM » Author: Medved
I think the "school rules" may come either from misinterpreting what the "hot restart problem" actually is,

or as

A mechanism to make sure the lights do not remain ON after all left the gym (or where the installation was).
The point for that is, once the brief interruption extinguishes the lamps, they may appear as OFF, so it is very likely people will forget to really switch them off when leaving during this dark period.
But when the rule says "at power interruption switch OFF all switches and turn them ON only after 15 minutes", people will build a habit to turn them all OFF immediately at first. When they decide to leave without remembering something was with the lights, they remain OFF even after the cool down time.
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sol
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Re: Probe start hot restrike « Reply #3 on: November 27, 2018, 08:49:39 PM » Author: sol
Makes sense. I couldn't figure out why there would have been a technical reason for this, but the people leaving before the lights come back on makes sense.

If the ballasts are stressed by the maximum OCV during a hot restrike event, I cannot imagine it would be a significant problem when you only have a hot restrike about a dozen times a year, with enough 'cool down' time between each. The perpetual heat that occurs naturally during operation would be far more detrimental to the ballast windings and even then will take years to make them fail. I don't think there was ever a ballast failure in their 24 years of operation. They are still there on a wall in the ventilation room, unfortunately all disconnected.
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wide-lite 1000
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Re: Probe start hot restrike « Reply #4 on: November 27, 2018, 09:56:16 PM » Author: wide-lite 1000
the 10-15 min rule may have been to protect the gear? when we had mh hi-bays still at work the people who didn't know anything about hid lighting would frequently try to rapidly switch the the light switches on and off when there was a power interruption thinking the lights would come right back on. occasionally this caused fried ballasts
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Medved
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Re: Probe start hot restrike « Reply #5 on: November 28, 2018, 12:53:47 AM » Author: Medved
Unless there is an ignitor pulser aid (with some 230V setups designed to operate US lamps,...), with a CWA (most probe MH ballasts) there is no degradation in the ballast at all, don'tworry. If something may degrade, it is the lamp when the auxiliary discharge is burning too long (as described above) and the resistor inside of the lamp running very hot (with 350V OCV it gets about 10x higher power dissipation than with the lamp burning). So again only within the lamp. But never heard of it being a real problem (I would expect it is designed for that as well).
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