Re: Autotransformers

Hi Andy,

> In response to your statement, "Just because some people cannot articulate the reason for a risk does not make the risk go away.". I could reply, "Just because some people have a gut feeling that there is a risk, does not mean that the risk is truly there".

But you did reply so, and it makes no sense because YOU immediately acknowledge the risk. Playing devils advocate is all right for philosophy but does not usually gather helpful sentiment in a technical discussion.

> So, while I agree with you, and I am not saying there is no risk, I do feel that if it is there, it ought to be explainable, unless we are to get into the para-normal.

> Your example of a machine having it's entire frame live also assumes that it is not grounded, which I would say is always a possible mistake, but should never be the norm for wiring up a machine.

The risk is real, grounds can float after installation too, someone else pointed out clearly the same scenario I did while my message was in the moderation queue. It is not the only possible failure but a pretty clear one and foreseeable by more than just my gut.

> If you don't ground your machine frame, then lot's of different events could cause the frame to be hot. The one you describe, opening of the Neutral(if neutral is bonded to the machine), but also any break in any of the hot wiring that then happens to touch the machine frame.

Yes, but while you can keep fingers out of your machine and keep the wires tight you may not have full control of the utility workers who may swap live and neutral one day. A work piece may fly out of a holding fixture and sever the ground and neutral conductors.

> In other words, you should not be using the neutral instead of the safety ground.

One should obviously not use Neutral for grounding and visa-versa but this in a way what you are recommending. Electrical codes in South Africa, UK and likely in the USA will all have something in them that requires a single point NEUTRAL connection to a protective ground on the SECONDARY side of every transformer. This is partly to have a known point for testing and maintenance of the ground and also to prevent ground loops by having unexpected load currents flowing through the protective earth wiring and keeping the installation equipment all at the protective ground reference, this is all mentioned by others as well. Without this one cannot have the protective function of an Earth Leakage unit as the live and neutral currents cannot balance (some will be diverted back via the earth connection).

In some situations the problem of an direct Autotrasnformer use is not too great as a simple motor or heater does not need to have a ground referenced voltage and it can float at whatever residual neutral voltage exists. But with CNC equipment that does not have comprehensive isolation (good OPTOs or fibre comms.) the controller will prefer to operate with a common at ground potential when controlling the power devices. This is why the common point on most autotransformer installations will get grounded, and after transformer isolation this is a good thing as all the power and control gear has a common ground. However if you have the mains neutral connected to the common you are voiding the electrical code, preventing use of Earth leakage units and posing a serious shock risk if the ground return is poor and the neutral is disconnected.

It pretty much boils down to the fact that the isolation is missing and so everything connected to the Autotransformer must be treated as live (and require the extra layer of isolation) and this includes your computer as they share the common neutral/ground.

Also touching one end of a 100-200 V transformer secondary while you are grounded that has nothing connected to the other end will not give you a 'shock', yes you will likely feel some slight tingling from capacitive coupling but this is seldom a health hazard. However touching just the live wire of mains or an autotransformer circuit while you are grounded will cause a current to flow to ground. There is a difference between these two scenarios and it is not a paranormal one.

While I am an electronic engineer and can maintain mains wiring and have studied electricals codes to prepare for a wiremans licence this is all just free advice, you should check with a registered electrician and then follow their instructions if you want to be risk free, a death or a fire with your installation would expose you to financial risk or other liability and that risk is not a gut feel or paranormal musings.

The reason for the electrical code is safety (fire, shock), compatibility and reliability.

On many list you would get bumped off as trolling, I suppose rules are more lax here but repeating the same question and not accepting valid (you can go test them, use a low watt light bulb to mimic the body) answers would get you hundreds of flames right back. You see a troll by definition is someone who asks questions he knows the answers to in such a way that clever folk are supposed to think him funny and newbie's are supposed to make long drawn out attempts to try and assist with something that does not require assistance. In your case you admit to a number of people that the risks do exist (see your reply to me in the second para. from the top) and admit conditions where the Autotransformer is not suitable (like before bridge rectifiers), acknowledge the difference between Neutral and Protective earth and dismiss the idea that a isolated transformer secondary will not form a current path to ground and other obvious failings of the direct wired Autotransformer. Yet you keep asking for proof, but are disinclined to do the tests to prove others wrong as they would not. So this paragraph is in response to your moderation statement (you should not moderate posts where you are involved, poor management style, and it is said that one should always reprimand in private). I can see the frustration of most of the people responding to you and most have bit their tongue and held their council, hoping upon hope that you are not trolling and just a few more explanations will get you to see the light.

While I am not a regular poster here and can forgive myself for trying to help, I assumed that trolling was not cultivated thought you had to have good grounds for the repeated questions. However I recalled mention somewhere that you have had this discussion before and then the fact that you moderated a message leads me to believe that your intentions are not 100% honourable. You took someone to task about personal comments, yet you accused me of mixing paranormal sentiments into this discussion, this is a personal attack (I'm not offended as one expects such things, just pointing it out) as my comments were polite and helpful.

My apologies to all for the long post, I have run out of stuff to say so you are safe from further advice.

Regards

Kalle
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Kalle Pihlajasaari, Johannesburg, South Africa