ESD flooring using aluminium tape stuck to the floor?

[treez]

Hello,
We have to use a temporary hut for some electronics work. There is no ESD flooring. The floor appears to be chipboard with a clear varnish coating. Therefore, I stuck a few overalapping 1 metre strips of 5cm wide aluminium tape to the floor in front of the bench where I stand and work. I soldered a wire to these strips, and then soldered 10 MegOhms into this wire in series , and then wired from the 10 Meg into a mains earth socket.
Is this a valid way to tackle ESD , and could it be considered dangerous, bearing in mind that we work on live mains now and again?


(we also have an ESD mat on the bench and ESD wriststraps connected to this mat)

 

[betwixt]

Chipboard (MDF or OSB) is not a good insulator, in fact it can be relatively conductive so I wouldn't have bothered. There is probably more danger of conduction through the floor being a hazard than ESD.

Brian.

 

[Kajunbee]

I would think that the varnish would be a pretty good insulator.

 

[treez]

Thanks, what is classed as the safe resistance from human hand to earth in order to mean that live mains working can be considered safe?

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2763825/

..this appears to say its 16mA. Sounds a bit high?

 

[betwixt]

I'm not sure that report is really accurate, 16mA is described as 'let go' AC limit but it doesn't say what would happen if you didn't let go!
As a guess, I would say a practical 'not get seriously injured' current is more like 2mA but even that could be quite painful. It isn't the IxR power that causes damage, it's massive overload of the normal internal electrical signals that operate body parts. Spasm and injury while trying to extract yourself is likely to cause far more damage than overheated tissue. There's an old saying "its volts that jolts but mils that kills!".

Standard 1M series resistance limits the current to around 0.25mA RMS in the UK which while certainly noticeable is unlikely to cause injury.

Regarding the insulating effects of varnish, there is no doubt it doesn't conduct electricity well but the coarse surface of a chipboard floor would need a very thick layer to cause any static risk. Don't forget the entire surface under the varnish is partially conductive so charges would be widely dispersed.

Brian.

 

[dick_freebird]

While the 10Mohm series may make the tape matrix human-safe,
there may be a substantial self-capacitance to it that makes it
an ESD menace to components in the machine-model vein. The
same reason ESD mats are a low (but nozero) conductivity and
not a simple aluminum slab, DC current limited or not.

Not a spark-from, but a spark-to concern.

I'd bet that a suitable conductive floor paint could be found, or
roll flooring, that would be better.

 

[ted2]

The relative humidity plays a large role on conductivity of wood-based construction materials. Therefore it varies greatly, and is not considered an ESD solution. Sometimes it is even too conductive, other times it is just too dry to conduct well enough. All depends on air humidity and other similar factors.

Aluminum is actually too conductive, and in general not an acceptable solution, either. It can even be dangerous, as in case a person touches for instance a mains voltage carrying wire, a grounded metal can cause a large current through the body!

A good ESD floor material is made of slightly conductive material, and with a ground connection (usually close to wall) to prevent accumulation of charge. There are many kinds of materials such as mats, floor plastic coating ("vinyl" stuff), and even wall-to-wall carpets which are designed for the purpose.

Depending on which country you are, there are usually many suppliers who have relevant materials. Just google something like "esd flooring options" and modify that search with country's name, if required.

 

[betwixt]

Treez is in the UK. Believe me - low humidity is NEVER a problem here
I live in the wettest part of the UK and we pray for days when it doesn't rain. Treez is in a slightly drier location but there is very rarely a static problem here anyway.
When I lived in the mid west USA it was completely different and the air was often very dry, there it really was essential to employ static protection.

Brian.