personal safety - preventing electric hazard

Question

I usually have AC mains powered devices/circuit on my desk, and I rarely taste AC current by accident:)

Could anyone suggest to me what kind of protection I should use for personal safety against electric shock? Would an isolation mat under my table be enough? What are the considerations when selecting a proper item and installing it?

Answer 1

There are many ways to avoid electric shock. Unfortunately all of them are dependent on your working environment. Following are some of the precautions that you can take care of.

1. Try to wear EH rated Shoes. This is definitely your first line of defense.
2. Isolation mat is also really good idea. It provides protection if you are using a conducting chair.
3. You should make sure that the power supplies that you are using has proper earthing connection. Always use a power supply that has 3 connections to mains i.e. live, neutral, and earthing.
4. Try to avoid doing any re-wiring work near your desk as it tends to leave open wires on your desk.
5. Always install circuit breakers switches near your desks. This helps in breaking connections in case of short-circuit or component bursting.
6. Never try to handle a device after it is powered on. This is the mistake that even experienced people do.
7. If you have to touch a device after its been powered on try not to touch it at two points. This would not complete the circuit and you will not get shocked.

Finally I must have "fear of getting shocked". This way you will always be careful around electric equipment.

P.S. If you use shoes or rubber mat, always use anti-static gloves while working on instruments that are prone to static electricity. I have actually damaged a processor just because of static electricity.

Answer 2

As with most electrical items and wiring schemes I've come across the most common method for preventing serious shock is a residual current breaker: -

It works by sensing the current in live and neutral and if slightly different (implying a current to earth) it trips. It does this magnetically like this: -

Principle of operation.

1. Electromagnet with help electronics

2. Current transformer secondary winding

3. Transformer core

4. Test switch

L live conductor

N neutral conductor.

All this information and more can be found here on wiki

Answer 3

1. Your brain :) Always be careful and focused when you dealing with mains.

2. Residual Current Device (RCD), also known as Residual Circuit Breaker

See Andy aka answer.

3. Insulation Transformer (aka Safety Transformer).

It's 1:1 (230/230V or 110/110V) transformer with improved insulation between primary and secondary windings.

Adventages:

• It separates galvanically your devices from mains. That means, if you touch to one wire - current will not flow to ground by your body. To get electrocuted - you will have to grab 2 wires, not only 1.

• It limits power delivered to your bench. Getting electrocuted is not only thing to worry about. If you accidently cause short circuit - less energy will come out before circuit breaker/fuse turn off everything. Typical short-circuit protection (fuse or circuit breaker) needs few milliseconds. Before everything is turned off - you can get hit by some molten insulation, molten metal etc. Transformer will reduce such risk.

Disadventages:

• Residual-current device (RCD, described in Andy aka answer) cannot work with insulation transformer. RCD protects from something that can't happen with insulation transformer, however if you grab 2 wires from RCD output with no transformer - you get electrocuted seriously, but RCD will turn off mains because current will flow from L to N and also to the ground. If you grab 2 wires from transformer - you get electrocuded seriously and nothing will stop it.

4. Kill switch (emergency turn off button)

Very useful when:

• high-impedance short-circuit happends and it may cause fire of thin wires
• something on your bench is burning :)

Answer 4

There are two main ways to get zapped. In your case, assuming you're working at a bench or desk, neither is likely to be affected by your choice of footware.

The first, and most obvious, way to get a tingle is to grab one line with one hand and the other line with the other. There is no obvious way to rule this out - presumably you thought you needed to put your hands where you did. The standard way to minimize risk is the Pocket Rule: Keep one hand in your pocket when poking around in live equipment. Even this is not foolproof - if you're bellying up to a chassis and making contact with your chest, well, that's just as good as using your other hand.

The other way to do damage is through ground. Line voltages are capable of taking an alternate path through ground (as opposed to hot and neutral). This is the sort of path that standing in water can produce. There are two obvious safeguards, and the previous answers have dealt with them. A safety (or isolation) transformer will prevent ground paths from occurring, and a residual current breaker (known in the US as a GFI - Ground Fault Interrupter) will stop any unexpected flow before it has time to do damage.