My ATTEN 858D hot air gun suddenly began tripping the residual-current device in my home office. Let’s investigate! 👇

One quiet afternoon I was watching TV with the misses when suddenly I got a notification on my mobile phone:

Homelab power alert on ATS!

WTF!? One, or more, of the power sources on the automatic transfer switch for the homelab, was reporting faults. I quickly got down to the home office to investigate and noticed the room was dark, apart from the homelab rack.

The circuit breaker for the home office had tripped, causing the ATS to switch over to its secondary source (the basement circuit). In the circuit breaker panel, I noticed that the earth flag was showing, meaning that the breaker had tripped due to residual-current — an earth leak.

Tripped fuse with residual current protection

After some troubleshooting, I found that the ATTEN 858D hot air gun was the root of the problem. As soon as I connected it to mains power, the circuit tripped again.

I wanted to understand what was happening and why. So I got my multimeter and measured the resistance between the hot and neutral wires to ground. Between hot and ground there was no connection, but between the neutral and ground, I measured 34 Kohms.

---- = 6.76mA

So there was a 6.76mA leak between the neutral and ground inside the ATTEN 858D hot air gun… Since its power switch turns only off the hot connection, it didn’t matter if the tool was turned on or off.

It usually takes 30mA to trip an RCD, and I measured just 6.76. But since I measured with the power turned off, the resistance could be lower with the power turned on. And there could be other appliances with slight earth leaks on the same circuit; I will investigate that in the future.

Wires connected to PCB inside ATTEN 858D handpiece

Inside the handheld hot air gun, I found something interesting, a black mark between two copper lanes on a PCB. Tracing the lanes I found that they were, you guessed it, neutral and ground.

The ATTEN 858D is rated at 700W, that is about 3 amps — going through these copper lanes on the PCB!

Charred mark between copper lanes on PCB, through microscope

I scraped the charred area and cleaned it with electronics cleaning spray, a cloth, and compressed air. I measured the resistance again between the neutral and ground wire — no connection. I plugged the mains cable back in — and nothing happened, the circuit did not trip.

Mark between copper lanes on PCB, through microscope

So I had found, and fixed, the problem. But why did it happen in the first place? I reached out to the seller on eBay, with a description and photos of the problem. They in turns contacted the manufacturer, which did not have any explanation as to what might have happened. They did offer me a $20 replacement PCB — which I declined. I think I will be looking for another, higher quality, hot air gun instead.

The PCB was quite dirty — you can clearly see the difference before and after I cleaned the charred area and the area around it. So one explanation is that current has slowly flowed between the neutral and ground wire for a while. And as it slowly got burnt, the resistance dropped, causing the current to increase until the RCD tripped.

Last commit 2022-08-06, with message: add summary and emojies to older posts