Dangers of mixing double insulated in earthed systems

[vk3em]

Hello Crew,

This is a general warning to all newbies who have not encountered this problem before.

If you have a system that is earthed, be very careful of using any double insulated items, as they often leak current to their negative output. This applies to two pin plug packs/wall warts, two-pin laptop switching supplies, etc. The problem is that these two pin devices provide a -ve DC output, which is sometimes assumed to be earth potential, but is not (as its insulated from earth) buts it amazing how many people think it is at the same potential as Earth. Below is a photo of a generic double insulated wall wart from a reputable company. There is nothing wrong with it, but when you measure the AC voltage between the -ve DC and EARTH, there is a 120 v AC potential, this is powered from 240v AC.

Although the amount of leakage current is low, it is enough to:
- Give you a tingle if you short the -ve DC and earth
- Send you to hospital if you report the tingle and work in an electrical utility company (they take tingles seriously)
- Blow up the front end of sensitive RF test equipment (that one cost the company $30k USD)

In my work shop, I run earthed power supplies and avoid double insulated plug packs at all costs, particularly DC output ones (AC outputs are normally OK).

If you have to, seriously consider earthing the -ve DC supply rail from the double insulated supply. I have, for example, spliced into a laptop PSU, and manually tied its -ve DC supply to earth. This is particularly important if you are going to connect it (ie: a laptop) via an earthed connection to test equipment (RS-232 serial for example). Not an issue for wireless naturally.

Many of you will already know this, but there may be newbies who have not encountered the problem before. No, I didn't go to hospital, but my colleague yelled out "oh, I think I felt a tingle" next to the safety officer. I remained silent of course! Apologies if this has been posted before, I could not find it when searching.

Cheers
Luke

[m k]

More dangerous would be mixing dry and wet areas with extension cord.

Luckily it's less dangerous every day when old 0-class devices are phasing out.

[Doctorandus_P]

There is nothing "dangerous" about "feeling the tingle".
Try this: Put a 10k resistor across your DMM lead, and then measure the voltage again. Your measured 101V will then probably drop down to a few volts or less, and from that you can calculate the current and impedance. The current will be very low (at least from a properly built wallwart.) Also, all "regular" electronics measurement equipment should be able to handle this without problems. It is true though that some (expensive) equipment designed for very high frequencies  can be damaged by such ghost voltages. But for a normal DSO it's perfectly fine.

Also, the secondary voltage is always floating (except from some filtering elements). It's perfectly fine to use for example two 12V wall warts and then put the output voltages in series and use it on your breadboard as a +12V and -12V power supply. Although for beginners I do recommend to use lab power supplies with adjustable voltage and current limit. Especially the adjustable current limit can often be the difference between "no damage" and releasing the magic smoke, and your circuit being damaged.

However, don't buy wallwarts directly from china (ebay / aliexpress / etc, they are often garbage, especially the cheapest ones). Wallwarts bought in "local" shops should all have gone through proper channels to ensure some reasonable safety standards.

If you want to be absolutely sure, then use the wallwarts rated for medical use. These have extra reinforced insulation, very low leakage current and are also built in such a way that if a part fails, it will not lead to higher output voltages. But this does make them more expensive of course.

[Siwastaja]

What utter bullshit. You are seeing completely normal and safe behavior and "warning" people against something they encounter every day and which poses no danger whatsoever.

If some particular device fails the safety compliance, you need to report that device to authorities and warn about the exact product. Crap from Ebay is a real risk. The problem is not something being double insulated or capacitively leaking legal amount of current, the problem is selling illegal and dangerous crap illegally.

Apologies if this has been posted before

Approximately once a week, as long as internet forums have existed. Yes, everyday somebody finds about the existence of switch mode supplies and Y capacitors for the first time.

[vk3em]

Agree its not dangerous to your health, it can be dangerous to your test equipment!

I can assure you, you can blow up some very expensive RF test equipment if you are not aware of this.....

Luke

[tooki]

You posted in the Beginners subforum. What kind of beginners are playing around with "very expensive RF test equipment"?!?

[vk3em]

In 1999, one of our junior RF engineers was tasked to make a series of measurements on a network analyser, with a laptop, and an RS-232 serial connection. Without understanding the consequences of earth leakage, they blew up the front end of the network analyser costing the company around $50k or so. Today, there are plenty of beginner's who have access to cheaper RF test equipment like the Nano-VNA, so I thought the problem was worthy of a mention to those who were not aware or not experienced enough to encounter it before (ie: Beginners).

Cheers
Luke

[coppice]

What utter bullshit. You are seeing completely normal and safe behavior and "warning" people against something they encounter every day and which poses no danger whatsoever.

Its not always safe. I saw someone push the aerial plug into of a TV once, feel a tingle unexpectedly, be startled and push the TV over. There used to be some popular models of TV in the UK that really gave quite a strong tingle. It seemed to be in spec, but I never found it very acceptable.

[EPAIII]

If you think 100 Volts from a high impedance source tickles, try 40,000 from a lower impedance one. Pin holes burnt in my hand and the bottom of my foot. And through my sock and shoe. Yes, it went all the way through ME! Not sure if it went through or across the surface of the probe I was using. The meter was a Simpson 260 and it was OK. They were built like a brick sh*******. Not the wimpy digital meters of today.

And no safety officer so no trip to the E-room. I did take it easy for the rest of the morning until that feeling of faintness went away.

[Jeroen3]

Did you know that if you plug many devices into an earthed extension cord, like a typical desk, which isn't plugged into a earthed socket, like in Europe, this small leakage gets added with all the other small leakages to get painful tingle!