Hi,

I have a question that I have often wondered about.

For a work-bench or similar area in which live circuits are operated without protective coverings many recommend using isolation mats or boots. If these are properly maintained it is easy to understand that touching a live part with such isolation is safe.

However as I see it, when using such personal isolation there is a larger hazard. Since the live circuit is open and the person in question is probing various parts of the circuit there is a relatively high risk that one touches simultaneously two parts of the circuit that are at a different potential. As I understand it in that case there is really nothing to protect you. You are isolated from earth so the RCD will not detect any leakage and you are basically just a load in the circuit.

On the other hand if there is a good path to earth the RCD will trip as soon as you touch one of the live parts and you are safe.


So my question is: Is it really safer to wear isolation material ? If yes what can be done to protect in the event of touching different potentials simultaneously ?

Thanks

 

I think that the answer is if you touch two voltages simultaneously (that have a common reference like the circuit board ground etc ) nothing can prevent you from getting a shock.
If you are isolated by a mat etc then touching one voltage only will cause no current to flow and you so won't get a shock.
If you are NOT isolated then you are putting your faith in an RCD somewhere down the line that you hope will trip. I've never done it but I imagine that you will still feel the shock, but it won't be lethal.
I keep a 1:1 isolating transformer (with output floating) on my bench to feed stuff under test rather than a mat. This has the advantage that inadvertent contact with a voltage and something else (anglepoise lamp maybe) also doesn't matter.

 

Hard to tell since you are talking generalities.

What specific operations and voltages are you talking about operating without safety covers?

The usual safety recommendation is to use a 1:1 isolating transformer for such work. In some countries it is mandatory.

RCd's are often not fast enough for protection if the contact is good.

 

Years ago, my high school electronics teacher made us memorize this:

There are old engineers, and there are bold engineers. But there are no old, bold engineers.

Unfortunately, it required 300VDC from arm-to-arm for me to take this to heart.

I've not been electrocuted since.

Reminds me of another quote (I think Ben Franklin):

Experience keeps a dear school, but fools will learn in no other.

The moral of the story is this: keep one hand in a pocket!

 

There are some risks which cannot be avoided. In these situations, attempts to idiot-proof may have the effect of actually exaggerating the risk. The only solution I have for you is to wear clumsy rubber isolation gloves

, or maybe troubleshoot from the safety of another room, via robotic arms and a camera.

 

I'm referring mainly to mains voltage 230V/415V circuits.

I guess that how fast and how much you will feel the shock when an RCD trips depends on the RCD sensitivity and triggering current. A shock with a 30mA RCD will be painful but with a 6mA RCD the shock should be mild enough.

I agree a 1:1 isolation transformer give better protection but still touching two potentials is not safe.

I agree also that the RCD will probably not protect you when touching two points at different potentials simultaneously since the current though the body will probably be much larger then the current to earth. I really shouldn't have used the word 'simultaneously', what I meant was that if you are isolated (with boots or mat or transformer) you will be able to touch any one point of the circuit without feeling nothing and hence maybe without even knowing that you are touching it. So if THEN you go and touch another part which is at the opposite potential you would get a lethal shock. If you are not isolated the RCD would have tripped when you touch the first point so there is no risk of touching two potentials. I think it is very unlikely that you touch opposite potentials at exactly the same time.

 

grg183, welcome to AAC.

It is a good subject you have chosen to start with, well worth discussing from time to time.
As you can see from the variety of the responses, others agree on this.
I, for instance had forgotten the rubber gloves worn by electrical company operatives joining live supply cables, so well said strantor.

There is also much worth in doing a forums search around the subject as it has been aired many times before.

 

I onece accidentally touched a 115VAC live wire while grounded. It tripped the GFI (RCD) but I still felt a pretty good jolt.

 

I wonder how much electrical isolation a pair of nitrile or latex gloves provides. Probably not much. Maybe if they were doubled or trippled up. Not going to test it. Someone should design tight fitting electrical isolation gloves that are not cumbersome and allow tactile feedback.

 

I understand the point. A ground fault interrupter might save your life, but...
there is no way to always be protected by a ground fault interrupter and 415 volt mains are incredibly dangerous. The way I say it is, "There is no such thing as an old, stupid electrician".

You must always operate as if there is no ground fault interrupter. I call it, "layered safeties". I have been on a roof, fixing machines, when I found out the 3 phase disconnect did not disconnect all 3 phases. My screwdriver was vaporized, but nobody was even tickled by the current because I had 3 more layers of safety between me and ground. Just last week, I refused to do a job until I had a backup man there in case I got shocked and needed somebody to restart me. This is so ingrained in me that I actually break out in a sweat if I try to work with less than 3 layers of safety. You have to adopt this philosophy if you are going to be an old man someday.

Keep one hand in your pocket. Bring your own dry rubber mat to stand on. Throw tools with worn insulation in the trash can. Never bet your life that the equipment is properly wired, you already know it isn't functioning properly or you wouldn't be there fixing it. Never bet your life that somebody else turned off the correct circuit breaker. Don't even bet that the circuit breaker is actually opening the circuit.

The list is much longer than I could write here. Think! Measure first, then start working! Know what is keeping you safe and know whether your safeties are in good condition. You are the ultimate source of your own safety and the ultimate recipient of your own mistakes.

 

While working on a very old house (ca. late 1600s and remodeled several times) I accidentally touched a BX cable while holding onto a plumbing pipe, and was frozen to the spot . The cable obviously had an internal short to the armor. Fortunately, I was able to eventually free myself. My arm did however feel numb for most of the day after that. An electrician had to come in and replace the cable for me.

BX cable (top)

 

Never bet your life that the equipment is properly wired, you already know it isn't functioning properly or you wouldn't be there fixing it.

oooh the stories I could tell. Nightmares actually. Disconnected live wires swinging around inside supposedly deenergized panels, longing for intimate contact with other circuits or human flesh - just the tip of the iceberg.

But the impression I get is OP does bench work, sitting at an isolated bench and probably probing around at component level on circuit boards. The boards are or could be already isolated from earth so little chance of conducting current through your body to earth, yet the danger remains between points on the board. The one-hand rule is almost impossible to abide by. I maintain that unless there are skin-tight uncumbersome isolation gloves that I don't know about, this is a risk which must be taken; it must be understood and remain at the forefront of the technician's mind.

 

I was already picturing a machine with a switching power supply, thus the boards you are working on are internally isolated from ground and the GFCI wouldn't save you. Strantor covered that aspect.

 

As others have said, it is a subject worth rehashing from time to time. I completely agree with #12 that your best protection is a very well-nourished sense of paranoia. Sometimes our very efforts to increase safety get people killed because they become complacent owing to a sense of safety that encourages them to be less diligent.

An electrician that swapped out a failed 440VAC transformer at the company where my dad worked did it while the lines were live because it was going to take hours for the electric company to power down the lines (owing to the other business serviced by them). When asked what all he did different when he worked on a live installation, he replied, "Absolutely nothing. As far as I'm concerned, every installation is live, even if I can see the disconnected cables."

 

"Absolutely nothing. As far as I'm concerned, every installation is live, even if I can see the disconnected cables."

Excellent point. Treat every hunter as if he were loaded.

Electrocution is only one hazard. A good face shield to avoid a face full of molten copper is a good idea too.

 

No one has yet mentioned locking switches.

These, too, are a good safety regime.
They allow you to take some control over your own safety from the action of others.

 

Hello,

A lot of companies use the LOTO procedure for safety:
http://en.wikipedia.org/wiki/Lockout-Tagout

Bertus

 

Hi,

I have a question that I have often wondered about.

For a work-bench or similar area in which live circuits are operated without protective coverings many recommend using isolation mats or boots. If these are properly maintained it is easy to understand that touching a live part with such isolation is safe.

However as I see it, when using such personal isolation there is a larger hazard. Since the live circuit is open and the person in question is probing various parts of the circuit there is a relatively high risk that one touches simultaneously two parts of the circuit that are at a different potential.

-------------------------------------------------------------------

There is an old rule to obey at the workbench with exposed live parts - keep one hand in your pocket. Otherwise you may not even notice your other hand is touching something earthed, and your chest cavity completes the circuit!

You'll still get a belt if you're wearing leather soled shoes - but you reduce the chance of the current stopping your heart.