LED failure scenario

Thread Starter

steveparrott

Joined Feb 14, 2006
36
I should know this, but... There have been several problematic cases where LED lamps are installed in outdoor low-voltage (12V) lighting systems.

One LED lamp fails and all the other LED lamps on the same run fail to light. If you remove the bad LED then the others work fine.

Unlike incandescent lamps that fail as an open circuit, LED's can fail and short the circuit.

I'm assuming that the zero resistance across the bad lamp is causing a massive increase in amperage. But, doesn't the voltage stay the same? If the circuit breaker hasn't tripped and voltage is still being applied to the other lamps, why don't they work? Has the voltage dropped?
 

SgtWookie

Joined Jul 17, 2007
22,220
If the LEDs are wired in parallel, the current will flow through the lowest resistance. Since the bad LED was shorted, all of the current went through it instead of sharing with the other LEDs.
 

Thread Starter

steveparrott

Joined Feb 14, 2006
36
Is this a case where the voltage across the leads of one of the good LED's could still read 12V? I've seen that with a nicked wire - the voltage reads 12V but the lamp does not light - almost like trying to power a 50W 12V lamp with a 12V watch battery?
 

SgtWookie

Joined Jul 17, 2007
22,220
It's different. The LED shorts out the entire supply. You are referring to an open circuit, which is the opposite condition.
 

Thread Starter

steveparrott

Joined Feb 14, 2006
36
Ok, in one of the failed LED scenarios, the secondary 25A circuit breaker did not trip at the transformer despite the short. Instead, the lighting fixture superheated and started to smoke! Wouldn't you expect the circuit breaker to trip before that happened?
 

SgtWookie

Joined Jul 17, 2007
22,220
I do not know how your fixture is/was wired.

I don't know if was UL approved.

It sounds like adequate protection was not provided. However, you have only given the most vague information about these LEDs, so I don't know what you expect to hear. Outdoor 12v yard lighting should be pretty low power. I don't know why your fixture would have superheated.
 

SgtWookie

Joined Jul 17, 2007
22,220
Sheldons,
You should know that those types of circuits are against the Terms of Service, paragraph 6.

Please remove the attachment.
 

Adjuster

Joined Dec 26, 2010
2,148
Ok, in one of the failed LED scenarios, the secondary 25A circuit breaker did not trip at the transformer despite the short. Instead, the lighting fixture superheated and started to smoke! Wouldn't you expect the circuit breaker to trip before that happened?
Depending on just how big a LED lamp we are talking about, the components inside it would more than likely overheat at currents a long way below level at which your 25A breaker would trip. The resistance of some of these parts (possibly even that of the wiring) might restrict the current to a level at which the breaker would stay on.

The voltage at the LEDs would of course collapse because of excessive voltages dropped in the resistance of the overloaded parts.

I would agree with SgtWookie that it sounds like there was not a lot of protection going on here: you might hope the lamp would have an fuse, or fusible resistor etc. suited to its rating to protect it. If repairing the system you might think about adding some in-line fuses.
 
Top