Bench safety

[imix]

Hi

I was wondering if there is a comprehensive source (book, training manual,...) for safety at the electronic workbench for building and repairing mains powered devices. I found (and read) quite a lot of vendor information, training material, standards and books on fixed installation safety. A lot of the things in these resource absolutely apply to the bench for sure.

For work at the bench I was not successful at finding much dedicated material. There are a lot of forum/blog-posts, articles, youtube videos etc. on selective subjects (e.g. isolation transformers, appliance classes or verification after repair). What I'm searching is something comprehensive that helps getting the overall picture from necessary equipment to safe processes. I currently have no idea whether the
 knowledge i acquired over the years is in the "sufficient for safety" or in the "just enough to be really dangerous" ballpark.

Has anyone a good recommendation?

thanks
Stefan

[sahko123]

In terms of dedicated resources I dont know anything beyond what maybe a company would provide there staff.

But for working with mains there is some simple principles that should be followed in a sort of hierarchy. Firstly know what your working with if it has high potentials, basically anything above 50Vac and 75Vdc (from the CE low voltage directive). If so a good rule of thumb is to cover these sections in a protective way either with tape or an up-turned lunchbox like Dave (EEVBlog) does. If you need to probe the section and cant cover it physically then either set up your probes semi-permanently so you dont need to physically touch anything your self while the device is powered up. If this is for some reason impossible then try only probing with one hand behind your back, and if this is not possible then you should just be very very careful and aware of your hand position at all times.

The highest level of safety can be gotten from working with high voltage gloves which generally isn't necessary unless your working with several thousands of volts where even air can become a conductor.

Another useful way of working with mains is to have one hand on the power switch on an extension lead/power squid so that in the event of the board failing you can rapidly turn off the board to prevent any more energy getting into the circuit.

Mostly the main safety concern when working with mains is be aware and careful, ppe isn't necessary and can in some cases impede dexterity and ability to be careful. You should be super aware of what equipmenent is and isnt mains referenced (any test equipment worth owning will be labelled clearly if it is mains referenced or not).

I follow most of these principles but mainly just exercise caution.

[rstofer]

Probably the best way to protect from mains is to use a Ground Fault Circuit Interrupter device to protect all mains connected test objects.  If you do manage to get a shock to ground, duration and amplitude will be limited.  Just like your bathroom, kitchen, garage and outdoor outlets (in US).  Required application dates in US:

https://www.fhfurr.com/blog/when-where-are-gfci-outlets-required

GFCI devices trip at about 6 mA of unbalanced current flow.  What goes out the black wire better come back on the white wire (US colors).

You can include your test equipment on the same protected circuit.  Remember that all metallic surfaces of test equipment will be connected to earth ground unless somebody has hacked the cord.  This includes the BNC connectors.  In my view, under no circumstances should test equipment be floated.  It would be hard to talk me out of that position.

As to where you can safely connect the ground lead of your scope probe, well, that's a totally separate discussion and requires a schematic to even begin to make recommendations.  What you can't do is just pick a point and ground it through the scope probe.

[capt bullshot]

There's one thing you need, everything else is optional:
Knowledge of what you're doing and from which situations and faults what kind of dangers result. Basic knowledge of physics / electrical circuits and which way they have the potential to harm you.
If you're e.g. repairing or working on mains related stuff, the sources of danger are quite more complex than all the rules that apply for "consumers" working with safety approved devices in their unmodified state. So just forget about the common booklets intended for end user devices and electrical installation safety, they won't tell you the right stuff that you need if you're working on the guts of a mains powered device.

Paired with healthy respect of somewhat higher voltages and powers, you'll be safe with almost every kind of equipment.
It's some kind of learning curve, you'll have some mishaps but if you start small enough, they won't harm you.

I wouldn't recommend blindly trusting any kind of safety equipment that someone / some book recommends, there's still enough room to make conceptional mistakes that will bypass the intended protection.

[Gyro]

Speaking of 'blindly trusting'... Get yourself a pair of safety glasses (and use them!). Devices self-disassemble with much more violence with mains energy behind them.


P.S. Given that this is your first post (welcome!) in the beginners' section. Seriously reflect on your level of experience / learning before diving into mains powered equipment repair. We can't do that for you as we don't know you yet, so must assume the worst case. If you aren't comfortable at any point, stop and analyse what you are doing. If in any doubt, ask. You're off to a good start in this respect.

[imix]

Thanks for all the great input! I will include them in my work processes where applicable.

Maybe I should give some more information about my background. I have a masters degree in a technical field with several semesters of physics and electronics. I studied a lot of these two on my own time in the past years. I've been doing repair of mains powered devices since I was a teenager (yeah, not really responsible, times were different and build many (ELV) electronic circuits for use at home and at work. Some months ago I started to work in a weekly repair-café. During this work, I discovered new stuff such as the test procedure for devices after repair according to VDE 0701 (I think this is specific to europe). This showed me once more clearly that I have gaps I don't even know of. With the much higher exposure to mains powered stuff and some things that need to be debugged under tension, I decided it was the right moment to reevaluate my state of knowledge and ask here.

[Gyro]

It's comforting to hear that you have a decent amount of knowledge and hands-on experience.

Your mention of repair-cafés triggered a though. One of my 'uncomfortable feelings' about such (admirable) enterprises is the potential for people turning up with more enthusiasm than knowledge and bodging repairs - I'm certainly not including you in this! I've often wondered about things like public liability insurance too.

With this in mind, one piece of safety equipment that would be very helpful is an insulation and continuity tester. This could be in the form of a PAT tester or an electrician style tool. It would at least give you the confidence that mains powered items leaving the café are basically electrically safe, at least for shock hazard.

In the UK (I don't know about Switzerland) the regs allow for equipment PAT testing to be carried out with a professional electrician style wiring installation tester - I'm thinking of something like a Kewtech KT35. These have being abandoned by electricians as they purchase integrated all-in-one Multifunction Testers (MFTs) and so can often be found cheaply. These testers will measure insulation resistances of up to 2G ohms at 250V / 500V / 1kV with a minimum  current availability of 1mA (BS7671 / EN7671 requirement), and Earth connection continuity down to a 20 ohm (0.2R resolution) range at a minimum test current of 200mA (again BS7671 / EN7671). These are much more reliable than ordinary DMM measurements.

These sort of testers are often available second-hand on ebay, and unlike most electrical test equipment, you can easily verify their operation with a DMM and selection of low and high value test resistors (they are typically specified to a few percent anyway).

Obviously such tests are not infallible, but give a reasonable degree of confidence that nothing obviously dangerous has been missed.

[jonpaul]

Bonjour cher Monsieur we have worked in SMPS, HV, lighting since 1970s,

1/ use large 1:1 isolation transformers on entire bench or DUT

2/ stand on rubber or plastic mats, wear non conducting shoes

3/ do not float instruments or defeat safety earth of mains

4/ any HV or DC bus takes a charge that remains for a time constant after power is shut down

5/ use variable mains transformers to run up voltage gradually ( Powerstat, Variac)

Hope that this helps

Bon chance

Jon

[Le_Bassiste]

Hi
I was wondering if there is a comprehensive source (book, training manual,...) for safety at the electronic workbench for building and repairing mains powered devices.

you could ask your local "Berufsschule" on what they have on their education plan wrt "Sicherheit am Arbeitsplatz". or maybe ask an electric company in your area, they most likely have at least one employee as "Sicherheitsbeauftragter".

[SmallCog]

I’m a believer in the 1 hand rule

If I’m probing an item that’s powered up with it’s case off my free hand will be behind my back, not resting on the items chassis

I actually asked the local technical college about what they teach in regards to working on powered devices and they don’t tackle the topic at all. They teach their students to never work on a mains powered device because “they’re not electricians” yet the same classrooms were once used to teach TV repair…

TAFE aren’t really preparing the next generation of technicians very well….

[imix]

As we have a very frequent (weekly) repair café, the people doing the repairs are all regulars. In my experience they all know their limit and ask other people if they don't know how to work on something. The problem is (probably) more devices being brought in that the owners already started to tinker with.

We have a BEHA Unitest 0701 at the repair-café which I started to use it after the repairs. Maybe I should start using it to do a test before starting to work on it to exclude problems from other people who did something with it before it reached my hands. The BEHA does not cover all the ranges you mentioned but I assume it should be ok as it is used for official tests in Germany/Switzerland.

The 1 hand rule absolutely makes sense. The point that they do not teach it seems to be somewhat general. I found a lot of teaching material for electricians but nothing along the lines we are discussing here.

So far I haven't found any tested/certified shoes/mats which are not electricians plastic boots (which I find to be be somewhat overkill). Maybe I should do some isolation tests with at higher voltages on shoes

[Shock]

On unknown devices you should at least confirm the chassis or exposed metal parts are at least grounded to the mains ground or totally isolated from the conductors. Then you can do a power on test if it was known to be powering on ok and not smoking or blowing fuses etc.

[BrokenYugo]

Whenever possible live probing should be avoided, make connections to a disconnected (unplug and have cord end visible as a confirmation!) DUT, then power up and observe hands free. I think this gives more time to evaluate the setup and catch any nasty mistakes. If you must poke around, use shrouded probes, DO NOT FORGET about the ground ring on a scope probe.

I second having the DUT on a 5mA RCD/GFCI at the bench, makes those oopsies to earth a lot less spectacular/destructive/deadly.

That said, I think rule #1 is to understand what you're doing, i.e. what you're working on and the tools you're using, do not proceed without solid understanding. All the other stuff just helps prevent death when mistakes are inevitably made.

[NaxFM]

I'd say that the best safety comes from experience.
One time I was probing a broken power supply connected to mains, I shorted two contacts and it exploded.
From that moment I wear safety glasses and gloves whenever I play with mains or dangerous voltages.
Of course i apply the usual rules: use only one hand unless it's absolutely necessary, never wear metal bracelets, rings or necklaces, leave the device on only for the required time to perform the test, always check high voltage capacitors with a multimeter before touching anything, use probes with shrouded tips whenever possiblem, if not using a differential probe ALWAYS connect the oscilloscope ground lead to a secure point of the chassis of the DUT.
I don't usually work with mains powered stuff, but if I did I'd surerly buy an isolation transformer and a GFCI