Safe way to play with transformerless supplies?

[Monkeh]

Oh, and have I mentioned you can't rely on one to save your life even if it does trigger properly?

nope you didn't mention it .. go on.... I want to know more about this ...

The current has to pass through you to trip the device. And it obviously isn't limited to 30mA. It's quite possible for bad timing or an unknown heart condition to lead to a fatal shock even when an RCD correctly operates. They merely reduce (okay, drastically reduce) the risk.

[codeboy2k]

Of course not. If you happen to touch both the hot side and the neutral side at the same time you will just become a load on the supply, a load which the supply will be happy to energize 

If the breaker tripped on normal loads between live and neutral nothing would ever work.

No I realize a person can be a load, but the human model I had in my head when touching both sides (after an isolation transformer and RCD) was that the human formed (basically) a Y shaped resistor network to ground two hands, and a body, and feet on the ground... there would be leakage to ground and the return current would not equal the source current.. but then I was shown the light when others pointed out that the current from an isolation transformer always wants to return to the transformer i.e. there can never be a path to ground (because it's floating).

And they are *very* *safe* .  It *will* trigger.  Hold the shank of a screwdriver and stick it into the hot side of a GFI/RCD plug while standing in water, you'll be fine  

Have you tried this experiment? I have (safely), and I can only say it hurts. 30 ms of mains AC is a hell of a jolt. I would not want to trust my life to it under unsafe circumstances.

not with a screwdriver, but I have touched a live wire at the end of a long RCD fed socket chain.  It tingled and tripped.

The GFI/RCD would still trip if there was leakage current (i.e. all the current flowing out does not come back in).

Have you heard of Kirchhoff's current law? All the current flowing out always comes back in.

I thought it was just an advisory

[IanB]

not with a screwdriver, but I have touched a live wire at the end of a long RCD fed socket chain.  It tingled and tripped.

The experience here depends on the contact resistance between your skin and the live wire, and the availability of a return path for the current. For me it has varied between a mild tingling "oh, that's live" at one extreme to "Ow! #*($@&(##!!!" at the other. It all depends... 

[codeboy2k]

The current has to pass through you to trip the device. And it obviously isn't limited to 30mA. It's quite possible for bad timing or an unknown heart condition to lead to a fatal shock even when an RCD correctly operates. They merely reduce (okay, drastically reduce) the risk.

they need to be faster then

joking aside, I think I get it now.. so after all this, the conclusion is that the GFI/RCD should not be relied upon to save your life, but having one there can signficantly reduce the risk of a catastrophic accident.  And they are no good on floating circuits such as after an isolation transformer, which would seem much safer for use on the bench than relying upon the RCD.

 

[codeboy2k]

not with a screwdriver, but I have touched a live wire at the end of a long RCD fed socket chain.  It tingled and tripped.

The experience here depends on the contact resistance between your skin and the live wire, and the availability of a return path for the current. For me it has varied between a mild tingling "oh, that's live" at one extreme to "Ow! #*($@&(##!!!" at the other. It all depends... 

I've touched mains probably a 100 times... always felt a tingle nothing more... certainly never Ow! #*($@&(##!!!" .... I guess I have  too much resistance

[Monkeh]

not with a screwdriver, but I have touched a live wire at the end of a long RCD fed socket chain.  It tingled and tripped.

The experience here depends on the contact resistance between your skin and the live wire, and the availability of a return path for the current. For me it has varied between a mild tingling "oh, that's live" at one extreme to "Ow! #*($@&(##!!!" at the other. It all depends... 

I've touched mains probably a 100 times... always felt a tingle nothing more... certainly never Ow! #*($@&(##!!!" .... I guess I have  too much resistance

Just a puny low voltage.

[brainwash]

For a real shocker you should have a solid return path to the neutral/ground. It can be either your other hand or normal shoes on concrete. It's still up to skin resistance though, some people are more 'gifted'; I personally know a few electricians that wire everything while live and just use the back of the hand to test for voltage. I don't have the necessary dry skin to try that.

A nice story (of what not to do) was told by a friend of mine: he was wiring up some country house doorbell at the gate. During this, he always felt small bites on his feet and was constantly checking for ticks, spiders, mosquitoes and such. In the end it proved to be the wet morning grass touching his feet over the rubber flip-flops (thongs?) providing the temporary return path to ground. Somehow he always has stories to tell about how he thought the safety was off but in reality it was not.

Oh, regarding the 'tripping' of the neighborhood fuse from that guy (PhotonVids), I could not find the video. He was drawing more than 100A from the network. Maybe he's here to tell the story, though that's normally something I would not publicly admit. It was caught on video though.

[Zero999]

The sink in a bathroom does not create a zone.

It's part of the wiring regulations which divide that bathroom up into "zones" where you're allowed to put sockets and switches.

The isolation transformer vs RCD debate is an interesting one. I think both RCDs and isolation transformers have their unique advantages and disadvantages when it comes to electrical safety. An RCD is good because it will trip if there's any leakage to earth but if you form a direct path between live and neutral and there isn't enough earth leakage it won't help, it's a mechanical component which could fail to trip and it's no good if you want to test your circuit with a oscilloscope. An isolation transformer doesn't rely on mechanical components which can fail and you can test the circuit with an oscilloscope  but it gives no protection if you form part of the circuit, even if you are a good path to earth.

On balance, I think an isolation transformer is probably most suited to this application because it ensures the whole circuit is isolated so you can connect it to other equipment such as an oscilloscope.

Using two back-to-back transformers provides a greater deal of isolation and a lower voltage supply but it occupies more room. is less efficiency, has a higher voltage loss and poorer regulation due to the impedance being higher.

You can use a shaver isolation transformer or buy a small isolation transformer quite cheaply. I made my own 100VA isolation transformer by rewinding a transformer I got from work because it was going to be scrapped.

[richard.cs]

I bought a 240 VA isolation transformer on ebay - just the transformer, no case connectors etc. It was about £35 including shipping from Germany.

I put it in an old battery charger case with a few extras. There's a neon connected between mains earth and the transformer centre tap, it doesn't pass enough current for a dangerous shock but it does light up as a warning if one side of the output gets grounded, either by a fault or intentionally through a scope or similar. There's also a lamp holder on the top in series with the output that provides variable current limiting, that can be shunted with a switch when not needed. Note that this current limiting is in the region of hundreds of mA depending on the lamp fitted. It doesn't make anything safer but it can protect components.

[Monkeh]

The sink in a bathroom does not create a zone.

It's part of the wiring regulations which divide that bathroom up into "zones" where you're allowed to put sockets and switches.

The sink isn't.

[Zero999]

The sink is zone 2.

[Monkeh]

The sink is zone 2.

Perhaps that non-official drawing claims it is, the regulations do not.

[brainwash]

I put a little thought into the process and came up with a safer way to do this.
One of my pending projects involves making a new uC board for a capsule coffee machine which works directly on the mains. The previous design used some Holtek chip powered at 5V through a zener diode. I already have the new board ready, based on MSP430 but I did not want to plug the thing up until I had a safe way to power it.
My idea is to have the board powered up (at least temporarily) from a 2xAA battery holder switched on by an optoisolator. The draw is less than 10mA so it should work, at least in theory. The old board with the SCR and everything is still in place, I'm just piggybacking on the uC pads.
If anyone has a better idea I'm open to any suggestions.
My reason for the thread was that last week I was trying to measure how the [hall] water sensor works and have gotten quite a shock. This was on an identical working machine but have no idea where I got the 220 voltage differential from. IMHO it's poor design especially for something working with/in water.

[Gigawatt]

Tun it off !

[SeanB]

I bought a 240 VA isolation transformer on ebay - just the transformer, no case connectors etc. It was about £35 including shipping from Germany.

I put it in an old battery charger case with a few extras. There's a neon connected between mains earth and the transformer centre tap, it doesn't pass enough current for a dangerous shock but it does light up as a warning if one side of the output gets grounded, either by a fault or intentionally through a scope or similar. There's also a lamp holder on the top in series with the output that provides variable current limiting, that can be shunted with a switch when not needed. Note that this current limiting is in the region of hundreds of mA depending on the lamp fitted. It doesn't make anything safer but it can protect components.

So basically you made an isolation transformer with a fault warning indicator.

[brainwash]

Tun it off !

What, the Senseo? It's probably one of the the most popular models in the world, so they probably got it right and I got it wrong.