Transformers, Isolation, Protective Earth/Ground

[JenniferG]

After watching Dave's video on Oscilloscopes & Protective Earth precautions, and reading about using a non grounded soldering iron to fix 3478a battery.. I have a couple questions about transformers, isolation and protective earth.  Hope you don't mind answering.

So from what I am gathering, if you stand in your bare feet on a wet floor and touch either of the secondaries of a transformer, current cannot go through you? Because those secondaries are isolated from ground? [You'd have to touch both to complete the circuit?] Also if a DC circuit is floating (i.e. no protective earth wired to chasis), and you are in your bare feet and touch the positive side only of that DC circuit, would current flow through you to the ground?

I understand the power from power company is grounded, all along the way, to protect line & equipment from things like lightning damage. It's also grounded right outside the circuit breaker of your home as well, right? (there is a metal pole behind mine going into the ground anyways).  I notice both neutral (white in the USA) and protective earth (ground) are wired to the same points on my circuit breaker for the house -- i.e. to that grounded metal pole. I understand how if a person touched the hot wire (black in the USA), the circuit would complete via your body, and via the ground, taking the shortest path (path of least resistance) to that metal pole behind my circuit breaker.   

Just trying to understand how a circuit could complete, through a human, through the ground with a floating DC circuit or secondaries of a transformer.

Sorry for my question, but I feel I will end up being smarter reading responses. 

[jeroen79]

For current to flow there must be a complete circuit.

A voltage source is floating when neither of it's terminals are not connected to ground.
This applies to AC and DC voltage sources, batteries, generators, transformers (secondary) and what not.

If you consider your body to be a resistor then your body can be used to electrically connect various things together.

For example, if you touch both terminals you complete the circuit and current will flow though you.

If you touch one terminal and ground but leave the other terminal untouched the circuit will remain open.
Only when the other terminal is connected to ground (direct or indirect) will there be a complete circuit.

Just draw things out to see if a particular scenario results in a closed circuit.
If it does not not current can flow and you will be safe(ish).

[JenniferG]

So you are basically confirming what I wrote then right?  I just need confirmation that my thinking is correct. 

[Ian.M]

One 'gotya' is devices with a two pin mains lead and a switched mode power supply (SMPSU).  The high frequency high voltage waveform on the primary tends to get capacitively coupled to the secondary winding, so a Class Y capacitor is usually connected from secondary 0V back to the primary side DC bus negative to shunt the HF capacitive leakage back to the primary side.   Unfortunately the class Y capacitor also passes a mains frequency leakage current.  The capacitor value is chosen to safely limit the leakage current.
However if many devices are interconnected, (e.g. a complex AV system), without any secondary side grounding, the class Y capacitors are in parallel and the total leakage current can reach dangerous levels, resulting in a shock risk.  The worst scenario is if you have a roof aerial connected to said AV system and have not run a ground to it, as a most unpleasant and possibly lethal surprise awaits anyone working on the roof.

[capt bullshot]

What really matters is the amount of current that goes through your body, the path through your body, and the frequency (consider DC as 0Hz). The voltage on its own does not hurt, even if it is 100's of kilovolts.

If you have AC (even with as low frequencies as the line frequency), current will flow not only through resistors, but also through capacitors. With DC, you get an inital charging current through the capacitor until it is charged to the same voltage as the source.

There's always a small amount of capacity from any loose wire to ground or any other surrounding objects.
So if you touch an open end of an isolation transformer, a tiny amount of current will flow from the transformer through your body through that capacitance back to the other end of the isolation transformer.Thats basic physics and electrical science. Because the capacitance is so small in value, and the voltage is low enough, the current will be small that it won't hurt, you won't even notice.

Do the same thing on the open end of an isolated 100kV transformer and you'll get fried from the capacitve current

So electrical safety reduces basically to one thing:
Any means to limit the amount of current that can go through your body to a safe value (< 5mA in most cases), often redundant measures like isolation and protective grounding of the containment. If the isolation fails, the protective ground will divert the current to ground and cause the circuit breaker to break the circuit. Usually there's also limited voltage involved.

[JenniferG]

Thank you for the replies. I am going to have to re-read everything a few times so I can grasp it all.

I have another example question, which is more simplified vs. what I was asking earlier.  Perhaps this might clear things up for me.

Say you had a plain transformer that had on its input neutral (white in the USA) and hot (black in the USA) with protective earth ("ground") wire not connected to anything.  And say you connect a 1/8 amp fuse to the end of  protective earth wire, which is in turn not connected to anything.

And say this tranformer just has one pair of secondaries connected to nothing.

You turn it on.

You connect either one of the secondaries to the fuse which is connected to protective earth.  WIll that fuse blow, or will it do nothing since the secondaries are "isolated" from the primary circuit & earth?

[capt bullshot]

No, the fuse won't blow. Assumed I understood your setup correctly: You connect one end of one of the otherwise non-connected secondaries to one end of the fuse and the other end of the fuse to ground.
There's no (low impedance) return path for the current, so this is considered an open circuit. The stray capacitance I wrote about is really     negligible here.

[capt bullshot]

I've read your original question again, I think now I've got the point

So from what I am gathering, if you stand in your bare feet on a wet floor and touch either of the secondaries of a transformer, current cannot go through you? Because those secondaries are isolated from ground?

No, no current cannot go through you, because the transformers secondary is isolated from ground. The transformer opens the circuit.

You'd have to touch both to complete the circuit?

What exactly is both? If you touch both ends of the secondary of the transformer, yes you'd close the circuit. Not if you touch the primary with one hand and the secondary with the other, because the transformer opens the circuit.

Also if a DC circuit is floating (i.e. no protective earth wired to chasis), and you are in your bare feet and touch the positive side only of that DC circuit, would current flow through you to the ground?

No current would flow, if "floating" means "isolated from ground"
Edit (add more confusion): "floating" does _not_ neccessarily mean "isolated from ground". It often means "a supply voltage measured between two points in circuit, superimposed with an AC or DC voltage and having a conductive return path to ground". In this case, a current would flow through you to ground.

I understand the power from power company is grounded,

Yes, if you touch the live wire, current would flow through you to ground. You close the circuit.

Just trying to understand how a circuit could complete, through a human, through the ground with a floating DC circuit or secondaries of a transformer.

That circuit won't complete, no current would flow. Except for:
- there's a failure in the isolation
- the isolation rated voltage is exceeded
- or other faults (maybe you dropping a loose wire into the circuit by accident)

So the concept of isolation for human safety is to introduce one more point (e.g. the isolation transformer) that opens the circuit in case you accidentally close the circuit using your body. If you don't do any accidents, the circuit is open anyway, so its redundancy for your safety.

[james_s]

Even at 50/60 Hz there will be *some* capacitive coupling, but it will not be enough to be lethal. If you were to stand barefoot in a puddle you may well get a tingle from touching one side of a transformer secondary though. Either way I would *not* recommend testing this theory. The isolation transformer drastically increases your safety but it's sort of like seatbelts in your car, use them, always, but don't try crashing into a wall just to see if they work.

[JenniferG]

Even at 50/60 Hz there will be *some* capacitive coupling, but it will not be enough to be lethal. If you were to stand barefoot in a puddle you may well get a tingle from touching one side of a transformer secondary though. Either way I would *not* recommend testing this theory. The isolation transformer drastically increases your safety but it's sort of like seatbelts in your car, use them, always, but don't try crashing into a wall just to see if they work.

Yes of course I'd never try any of this, not even the fuse on the protective earth with secondary wire touching it to see if fuse blows.  I was just being hypothetical so I can try to understand this.

Transformers are pretty amazing.  Can't wait to learn more about them in detail.

I appreciate all the help guys, thanks!

[bitseeker]

In one of the other threads about isolation transformers, someone once posted the following link:

http://horologyzone.com/diyer/topic/isolation_transformer/isolation_transformer_safety.html

It has a number of illustrations of scenarios with and without an isolation transformer and how/when current flows.

One thing that isn't clear is where the RCD/GFI, which is mentioned in some scenarios, exists in the circuit. It sounds like it's on the primary side, not on the secondary side, of the isolation transformer.

[pitagoras]

Your understanding is correct.
For higher voltages than mains you should normally consider what "isolated" means, start to pay attention to isolation ratings of tools, instruments, etc., even the air.