Replacement fluorescent ballast issue - light keeps turning off

Question

TL;DR

I have replaced a ballast for a 2x70W T8 strip light. The light illuminates for <5 minutes before turning off. Flicking the light switch turns the light back on, again for <5 minutes, before switching itself off again. Have I selected the wrong ballast, wired it incorrectly or is it faulty?

Full details

I have three Wirefield BAS270/HF twin-tube flurecent strip light (T8) units which are about 5 years old. One unit has stopped working. After swapping known good bulbs and basic multimeter tests I determined it was the ballast that had failed. I asked Wirefield if they could supply or provide details of a suitable replacement ballast, however they couldn't help as they are now "obsolete".

The original ballast doesn't appear to have any manufacturer's mark. I assumed dp was the ballast manufacturer but I cannot find any relevant Google results, therefore I purchased a Tridonic ballast, matching the specification as best as I could.

The bulbs are OSRAM LUMILUX L 70W/840.

Failed ballast

Replacement ballast

Tridonic 87500523

The replacement ballast has 6 wires whereas the original had 7 wires (pin 7). From the wiring diagram, it appeared to be surples to requirements, so I removed it.

Each bulb "socket" has four electrical connection points (depicted by the circles). The original was wired thus:

I rewired it, removing the wire shown in red:

The light powers on, but only stays illuminated for about 5 minutes. I can wait 3-5 seconds, flick the light switch and it illuminates again, but then turns itself off after about another 5 minutes. The pattern continues.

I notice the light with the replacement ballast takes 1-2 seconds to start, whereas the originals are instant.

If anyone has any ideas, I'd be grateful if you could share your thoughts, please.

Answer 1

You did everything right

I presume that your double circles mean that each pin of the lamp-holder socket is connected to 2 terminals. (so 4 terminals per lamp holder). That's exactly how it's done in North America also.

Your diagram showing the wiring changes is exactly correct. Assuming the greenish wire on the far left didn't get jarred or messed up in an unexpected way.

The ends of the tubes contain "preheat filaments" similar to an incandescent bulb. (don't worry, they don't burn out). They are intended to be run for a second or two before striking the arc inside the tube, to ease wear and tear on the tube. After the arc is struck, the pre-heaters are irrelevant and are simply used (both pins together, or simply one pin) as high-voltage anodes.

Cheaper "instant-start" ballasts don't bother with pre-heating, and simply strike the arc on a cold tube with higher voltage. However that is harder on the tube, and means replacing tubes more often.

Conversely, the best "programmed-start" ballasts run the preheat filaments as long as it takes for ideal start conditions, and these are so gentle on the bulbs that they can be on motion sensors, e.g. in bathrooms.

All that to say, the momentary start delay is money in your pocket. It sounds like you stepped up from a cheapie ballast to a better quality unit. That's often the case; fixture manufacturers often get low-bidder ballasts.

Anyway back to your troubleshooting: you should see all four preheat filaments glowing during your startup sequence. Note that the old ballast might have been able to support 1-tube operation; the new ballast cannot because it needs both tubes present for the left side pre-heaters to work.

For a fixture to work properly and then cut out 5 minutes in; that has nothing to do with pre-heat, and the wiring is clearly good if it preheats and strikes. It may be a thermal issue of a component inside the ballast ??? Or a thermal issue with a wire connection.

If you are positive on the wiring, the next step is contact the manufacturer.

Then there's LED

People will be unhappy if I don't trot out the LED option. They make LED replacement "tubes" that run directly on AC mains; you rewire to bypass the ballast and put AC power on the lamp holder(s). Some put line and neutral on opposite lamp holders; others on the same lampholder which I think is pretty shady at 120V even. I find most of the stock to be not very well made. So you have to watch what you buy, but it's a supposedly "permanent" solution, and although LED got a bad introduction with harsh, blinding blue-ish light of terrible CRI, you can now get LEDs of every style that are gentle, high-CRI and any color temperature you want.

I for one am perfectly happy with real fluorescent; I can get reliable ballasts and tubes from established suppliers like Philips and GE; ordinary hardware-store tubes are better CRI than LED "tubes" in the same shop, and certainly better reliability.

Answer 2

To draw a line under this. I eventually got around to ordering another Tridonic 87500523 ballast (the same make and model as before) and to date, no issues, so it looks like the first replacement was faulty.

Interestingly, it still has a 1-2 second start delay. It is 3 of 3 at the end of a radial circuit. When I performed a soak test, connected to a 16 amp supply, it started instantly.