Are there any adverse effects from isolating the AC power supply on my electronics bench?

Hi Mike, why do you want to do this? Mains powered equipment needs an earth for safety and emc filters. Portable equipment Safety Transformers generally have a centre taped "isolated" secondary (i.e. not an auto-transformer) but the centre tap is earthed...Peter

Hi Peter, Mike asked me to respond as I am helping him set up the bench and the isolation transformer is my idea. Learning is part of any activity and I always keep my mind open and try not to let my own ideas become hardened. Both Mike and I use our benches for servicing electro-mechanical equipment, specifically dental equipment, as well as experimenting with electronics. This includes x-ray controls, timers, motor and actuators control circuits and electronic interfaces with the real world. I originally installed an isolation transformer on a series of outlets in my own shop so that I could run a couple of Variac power supplies without the danger of having direct connection to the mains. I also from time to time like to be able to connect my scope probe ground at a higher potential point in a circuit. In addition to the isolated bench outlets I also have standard outlets that can be used if there is a need. I am not an engineer but rather a self educated technician and I decided it would be a good idea for Mike and me to throw our isolation transformer idea out on the forum so we could learn from members like mcb1 and yourself who are engineers. Mark has already given us some good insights and options that will cause us to modify our original plan. Any additional insights you can add will be appreciated.

John

Hi Peter, Mike asked me to respond as I am helping him set up the bench and the isolation transformer is my idea. Learning is part of any activity and I always keep my mind open and try not to let my own ideas become hardened. Both Mike and I use our benches for servicing electro-mechanical equipment, specifically dental equipment, as well as experimenting with electronics. This includes x-ray controls, timers, motor and actuators control circuits and electronic interfaces with the real world. I originally installed an isolation transformer on a series of outlets in my own shop so that I could run a couple of Variac power supplies without the danger of having direct connection to the mains. I also from time to time like to be able to connect my scope probe ground at a higher potential point in a circuit. In addition to the isolated bench outlets I also have standard outlets that can be used if there is a need. I am not an engineer but rather a self educated technician and I decided it would be a good idea for Mike and me to throw our isolation transformer idea out on the forum so we could learn from members like mcb1 and yourself who are engineers. Mark has already given us some good insights and options that will cause us to modify our original plan. Any additional insights you can add will be appreciated.

John

If this isolated bench were in a corporate lab you would have to do a risk analysis and you would be restricted to who had access to the bench. It is dangerous to run live equipment with no earth and it would be treated as using live equipment. You would be restricted to using safety connections and told to attach connection with the power off unless there was some other protection in place. Earth is your protection and electrocution is painful or/and deadly. Equipment with emc filters will charge up the chassis if it is not earthed and will be ineffective.

When you say "run a couple of Variac power supplies without the danger of having direct connection to the mains", what is less dangerous, you should ask yourself? It sounds more dangerous to me as you may be lulling yourself into a false sense of security.

Now when you say "I also from time to time like to be able to connect my scope probe ground at a higher potential point in a circuit" you hit on a real issue. The solution is best to use a isolated/differential input scope. These are available from Tektronix eg. Digital Signal / Digital Phosphor Oscilloscopes (DSO/DPO) | Farnell element14 ...these are the scopes you would find in a corporate lab...you can buy second-hand.

...or at lower bandwidth but more affordable from pico scope eg. PICOSCOPE 3425 - PICO TECHNOLOGY - PC OSCILLOSCOPE, DIFFERENTIAL | Farnell element14 ...this is the scope I use at home  as I need isolated channels.

Floating an ordinary scope off earth is not safe, the isolated scopes come with safety leads.

Also you say "In addition to the isolated bench outlets I also have standard outlets that can be used if there is a need." If you mix earthed and non-earthed equipment, you may get paths to earth back through test equipment earth, and potential for smoke! I've seen that happen.

What you are suggesting is a step back in time to the 1980s or early 90s when the only way was to use these risky non-earthed methods of working. And re-read what mcb1 has said...@mcb1

@peterjcs23

I'm not sure I would agree with all your conclusions.

While I'm sure the corporate world may place some other restrictions, this discussion is about what those 'unseen' risks might be, and is a very good way to emphasis safety.

unless there was some other protection in place

The suggestion to use an RCD provides exactly the protection against coming into contact with live equipment.

However since it is floating, then there is no potential, and the EMC filter will have no potential difference to earth, regardless of capacitors or not.

I have on occasion wanted to connect scope probes to voltages above ground, and had to use a capacitor to isolate the ground.

I'm not sure I would recommend using isolating transformers to allow this, since you have now made the chassis of the scope at one potential, while other parts of the circuit are at another.

This to me is a risky proposition, and the warning "... It sounds more dangerous to me as you may be lulling yourself into a false sense of security ..."  is exactly what I was thinking.

The link to the scopes is fine for voltages upto 600v, and I'm not sure what voltages you guys want to put a scope onto, but anything approaching that suggests you are way past hobby and into some serious high voltage work that requires full safety equipment. (High Voltage gloves, boots etc)

A isolation transformer is not a substitute ...

In addition to the isolated bench outlets I also have standard outlets that can be used if there is a need

Like Peter said these should not be available in the same area ... it's either isolated or not.

I've seen similar protential poblems when two different phases were on outlets near each other.

Two seperate peices of equipment were 480v apart and if one lost the neutral then it was a receipe for diaster.

Mark

Hi Peter, Mark, and Michael,

Thank you all very much for taking the time to address this question for us. I have learned some new perspectives and as a result we will be able to put Mike in a better position safety wise than I would have been able to do on my own. Just a clarification is that the Earth ground is carried forward in all of the isolated outlets but neither of the secondary leads are attached to it. Basically the (Line - Neutral) arrangement of conventional USA mains is exchanged for the floating mains voltage secondary. One of the things that I learned and will act on is that putting multiple pieces of equipment on the isolation circuit increases the chance that a fault to ground in one of them will go undetected and cause the system to revert to a (Line - Neutral) system unexpectedly. Mark,s article about one item on an isolation circuit at a time makes good sense.

One of the types of equipment that we service is an ultrasonic cleaner. The dentist places his instruments in a bath of disinfectant which is subjected to 25kHz to 30kHz of ultrasonic vibration for 15 minutes prior to sterilization. The circuitry of these devices typically come directly off the mains with no isolation and operate in the 120 to 170 VDC range. I have found the isolated supply is helpful in servicing these units.

As a summary of what I have learned we will rely primarily on what we call GFI circuits and use the Isolation Transformer only for individual situations involving the service of specific types equipment.

Thanks Again

John

I'm glad the discussion was useful. I was thinking that you could simply use the earth if you need to. In fact it is useful to connect earth to the isolated neutral as then you can guarantee they are the same voltage something that in a normal building scheme you cannot guarantee and for some test situations that is a good thing. But as you say, you need to think carefully about each situation; in a corporate environment we would carry out a risk analysis for each case, perhaps you should do the same...Peter

In fact it is useful to connect earth to the isolated neutral as then you can guarantee they are the same voltage

That would negate the isolation and the very purpose for doing it.

The whole idea of the isolation is to ensure that unintentional contact with the phase or neutral conductor (or any voltage from either) does not result in electrocution.

Mark