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Fuel for Thought

Officer, I swear it was working when I left earlier...

By Greg Huggins
Posted Mar 1st 2020 6:52AM

After you perform your pre trip inspection, you may do this.

You may do this to warn passing motorists about an accident ahead.

When being passed, you may do this many times during a day to alert a fellow driver that they are clear of you and that they can safely move back over into the right lane in front of you.

Yes, I am talking about flashing your lights, or turning them on and off.

Have you ever noticed that most often when you have a headlamp burn out, it is usually when you turn them on? While they can burn out during normal usage, more often than not, they will burn out when switched on (and there is a reason for that).

Incandescent bulbs work by flowing electrical current through a filament (tungsten). As the current flows, the filament glows to produce heat and light. It is precisely these components that also cause it to ‘burn out’. As the filament goes from cold(off) to hot(on), the heat produced will weaken the filament over time, causing it to become brittle and weak. When a headlamp (or any incandescent bulb) is first switched on, there is a surge of power (called inrush current) which increases the resistance to the filament and a weakened filament (from being cycled on and off) will not be able to withstand the electrical current applied and will cause the filament to break (burn out).

You may have done your proper pre trip inspection and all was well, but then you climb back into the cab and turn off your headlamps, notate your logbook or ELD, check your directions and then turn the lights back on and head out. Depending on the time of day, you may or may not notice that when you turn your lights back on to leave a bulb burns out at that moment. Dang it! I just pre tripped and it was fine. 

It was the cycling of the switch, turning them off and back on, that may have caused it.

After a pre trip inspection, do not turn the lights back off, if you do, you need to walk around and check them again.

For many years it has been common for truck drivers to help their fellow drivers to know when they are clear to move back over by flashing your headlamps once or twice. Every time you turn your headlamps off and on, you are one less cycle away from burning out the bulb(s).

I hope this explanation of how and why incandescent bulbs burn out helps you to keep yours working longer.

This may also help explain why the law enforcement officer still writes you a ticket for that burned out light. Since most incandescent bulbs burn out when switched on, the officer is less likely to believe it burned out after your pre trip inspection and that you should have seen it before you started driving.


See you down the road,

Greg