Autotransformers

Hi All,

Greetings in the new year from sunny South Africa. I'm just recovering after a lightning induced surge toasted an ADSL router, a PSU and a motherboard that was directly connected to said router via a UTP cable. The other 4 computers in the house all connect to the router over WiFi and did not suffer. The router shows arc damage on components at the one used UTP port. I will be fitting a WiFi card into the last computer when I replace the MB. The moral here is that all sorts of things can happen and sometimes do, this has happened to me before (twice) so I have been installing WiFi cards over time, this last victim was a lovely new low powered Atom computer that I was looking for a special PCI-Express WiFi card to keep the one PCI slot free, I knew better.

Just because some people cannot articulate the reason for a risk does not make the risk go away.

As for Autotransformers one can use a simple rule of thumb on when they are ok to use. If you are able (no voltage on the neutral) and comfortable (have faith in all upstream electrical networks and workers) with shorting the neutral to a good protective ground where the Autotransformer is fitted then it is indeed the same as an earthed isolated transformer but only if you actually DID connect the neutral to ground (and regularly opened the link to test for any new neutral voltages). If you did not do so because of the multiple code compliance violations, residual neutral voltage or suspicion about whether or not the Neutral would always remain at ground then you are taking risks. Much the same as I do when I connect an ADSL router to a mains powered PC via a UTP cable. For 18 months nothing happened, now instead of a WiFi card, I also need a new motherboard and PSU. With the Autotransformer if the Neutral is not firmly tied to Ground then the outputs can swing to any old voltage depending on what happens upstream and downstream.

Just so it is clear to all how easy a simple fault can make this into a disaster, all you need is the neutral to break before the transformer. Your CNC machine will STOP. You will use a multimeter to probe for voltage at the mains terminals and read 0V, you then stick your hand in there to wiggle that loose connector only to discover that EVERY metal bit of your machine is now at LIVE potential and you loose some skin as your hand bounces around in the control cabinet as you yank it out, if you are on damp concrete with no shoes and cannot get your hand out then things will not work out too good for your family.

Here in South Africa plug points are all protected by an Earth Leakage unit (RCI or something like that in the USA). It reduced the number of mains fatalities by some impressive figures during the years it was introduced, it is very valuable here because the ground is rather dry and the protective earth is not always good enough or badly connected. They are set to trip at 20mA Line-Neutral imbalance which will usually save your life. They also trip from poor wiring and equipment long before it become dangerous. I would not permanently fit an Autotransformer after the Earth Leakage unit and hope that it would be able to provide me with full protection after the transformer.

In the USA there exists the option of having a 220V plug point in some places where NEITHER power wire is at Ground potential, in effect it has two counter phase 110V Live wires, hooking this up to an Autotransformer is also NOT a good idea.

So to recap; if your Autotransformer has the Neutral firmly grounded you are OK for now, if not, keep your insurance paid up.

An isolation transformer either before or after the Autotransformer is a good solution. I used a Variac and a big 48V output transformer with a bridge and cap as my variable bench supply for a long time and miss it at times.

Regards

PS. Please edit out extra quoted text from your replies especially when you are in a heated discussion, the ritual will calm you and place you immediately on the moral high ground with the readers.
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Kalle Pihlajasaari - Johannesburg, South Africa