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Why do we use unshunted tombstones with Internal Drive T8s

ReneSola America

2017-03-08 13:36:22


Why do we use unshunted tombstones with Internal Drive T8s

If you’ve purchased our internal drive line voltage powered T8 tubes in the past, you know that we ship them with a pair of unshunted tombstones. But why can’t the tubes use the tombstones already installed with the fluorescent ballasts? More likely than not, those tombstones already in the troffers and fixtures are shunted. The difference between a shunted and non-shunted/unshunted tombstone is that the metal contact inside the shunted socket is one solid piece, giving it continuity if measured. Unshunted tombstones, on the other hand, have separate contacts for each pin, and were developed much later as fluorescent ballast technology advanced.

There are three types of electronic fluorescent ballasts in the market: instant-start, rapid-start, and programmed-start. All were developed to make T8 lamp life last longer or to consume less electricity than magnetic ballasts which uses electromagnetic induction to generate current through the tube. Instant-start and rapid start ballasts use the shunted tombstones as part of igniting the mercury-vapor gas contained in a fluorescent lamp. They heat the contact inside the tombstone to an extremely hot temperature which then cascades down into the tube to excite the gas. Once the gas gets dancing enough, an electrical current passes through the tube and it is from this excitement that the gas releases energy to mix with a phosphor coating on the glass of the lamp to make light that humans can see. In fact, the reason why ballasts are needed in the first place is to make sure the gas doesn’t get too excited to break the lamp. The problem, though, is that this ignition leads to the lamp burning out faster.

That’s the main difference between instant-start and rapid-start ballasts (programmed-start ballasts are a much smarter rapid-start ballast). Rapid-start ballasts slowly heat up the contacts and then slowly take their foot off the pedal once the contacts are heated up enough to get lamp to turn on. This can lead to almost double the operating hours of a fluorescent lamp compared to instant-start ballasts. And this is why new tombstones had to be designed. These are what are called unshunted tombstones.

Rapid-start ballasts don’t set their sights on heating up the cathode contact blindly. As the heat ramps up to make it easier to conduct electric current, they apply more electric potential (voltage) to the fluorescent lamp pins. The simultaneous heating and voltage drive is the reason the contacts need to be discontinuous and are called “unshunted”.

Our Internal Drive T8, High Efficiency Internal Drive T8, and our Dual Function T8 operating on line voltage all take advantage of the discontinuous contact to create a closed circuit beginning from one pin travelling down the LED chip board in the tube and out the other pin on the same side. This is what is known as “single-ended” or “single-powered” in the industry.


These T8 tubes can only use unshunted tombstones to avoid short-circuiting the tube and causing pre-mature failure. The other end of the tube, despite having metal leads, does not need to be wired. The pins on this end are more or less a placeholder/lamp holder and keep within the IEC standards for fluorescent T8 lamps dimensions and material construction. If we only need to use one tombstone, why do we provide you with two? We provide a pair of unshunted sockets not because both ends need to be wired but to avoid tombstone misalignment issues if you were to install only the powered end with our socket which can be of a completely different spec of the pre-existing one.


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