Shock from a block heater?

Don't swap out your GFCI or very bad things can happen...

Hey Mike,

The last few days, I have been feeling a slight zap from my pickup truck when plugging in my block heater. Today it was worse, I could barely hold on to the door handle getting into the truck. So needless to say I didn’t leave it plugged in. Could it be shorted out somewhere? Is this dangerous for my RV trailer?

BTW: A few months ago my brother-in-law swapped out the GFCI outlet in the garage where I plug in my block heater. It kept tripping and he said I didn’t need it for a truck. Was he correct? - Stan P.

Hey Stan,

Well, your Brother in Law created a very dangerous and potentially deadly situation. Let’s look at a possible sequence of events.

1. You initially had a properly functioning block heater that was plugged into a GFCI protected garage outlet which was grounded. And your extension cord had an intact safety ground conductor between the male plug and the female connector that plugged into your truck’s block heater.

   

2. Something happened in your truck that caused over 5mA of electrical leakage in your block heater circuit. This was most likely due to a corroded block heater element which allowed coolant to contact the internal nichrome wire.

3. The heater element coolant leak created enough fault current to trip the GFCI, but not a short circuit that would trip the circuit breaker. But the safety ground connection between the garage outlet and your truck chassis was still intact and protecting you from shock.

4. Your Brother in Law swaps our your GFCI outlet for a standard outlet. This removes a layer of protection, but your extension cord ground conductor is in place so you’re still safe from shock.

   

5. Something happens that breaks your safety ground connection. Could be that your extension cord ground failed or the ground pin was broken off. Your second layer of shock protection is now gone.

6. Both the GFCI and Safety Ground systems have been removed, and your block heater is creating a leakage current up to a few amps through the coolant. I’ve actually measured this current in my lab. So touching your truck while it’s plugged into a non-GFCI outlet can shock you with potentially deadly results.

7. When your truck and trailer are hitched together, any hot-skin voltage on your truck is now transferred to your trailer. This hot-skin voltage and current won’t damage any of your RV electrical systems. But because your trailer is bonded to your truck, the trailer hot-skin voltage (and current) can be dangerous for your family (including the furry ones) to touch.

Tomorrow I’ll show you how to measure your block heater for electrical leakage.

Let’s play safe out there… Mike