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

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

Mike

You should not need MOV's since its the same problem when you switch off the mains on the appliance ... the energy has nowhere to go.

Placed across the secondary terminals, they wouldn't cause any additional issues, so maybe you could include them in a box with the big red disconnect switch.

Just do helpful as there is no real correct answer.

Accident

That is a serious accident and thankfully the outcome wasn't different.

I'm glad it had modern safety equipment, and that is something I made sure of for our kids ... at least they have the best chance if it happens.

I know from many years of riding larger motorcycles you often avoided danger, whereas in a car you were a passenger waiting for the crunch.

18 and the course is about all you can do ...good choice.

Mark

I have not been able to keep up with this but one thing I noted (That im not sure was addressed) was that the secondary side had a ground connection through the GFCB (Ground Fault Circuit Breaker)  this would then present the same shock ability if you touched something on the secondary side of the transformer and effectively negates the need for the transformer.

The isolation provided by the transformer is designed to allow such things as a grounded scope to be attached to a "Live wire" without frying your scope, as there is a now common ground you have not achieved this

The ground fault protector will not work of course if there is no ground as there is no alternate route to close the circuit of the secondary side if it is not grounded

When this is on the Mains side of the transformer it is working by measuring the difference between the two legs of the mains wires (Live and Neutral) this does not matter if in the UK or Canada, 240 or 110, we all share a common theme, the neutral is connected to ground at the transformer and sometimes also at the fuse panel, the idea behind the GFCB is that id the current is passing through the ground wire instead of the neutral back to the panel ground it will trip the breaker as the neutral line is not balanced with the live anymore

this is the Canada / USA kind of wiring to show my point

and this is the UK version (Or one possible version) I think this also applies to Australia and parts of Europe

So what is common is that for an GFCB to work it needs a path back to its source so adding an isolating transformer provides the isolation to allow you to break that return path and provide a some what level of protection to you as the unit is now isolated. But if you add a GFCB back in and ground it (Required to make it work) you loose the point of the isolating transformer unless you start constructing a "FLOATING EARTH" for the purpose of the GFCB but that's kinda defeating the point.

This is a simplified circuit of a GFCB and as you can see, it needs the ground return to work

I think in summary, if your after personal protection, then you dont need the transformer but do need the breaker (The lower current device (5mA) would be the less painful but either will save your life in a crunch )

If you want to isolate equipment so you can safely probe around with your scope etc then you need an isolating transformer but you sill need to be aware that the scope or other equipment is also connected to mains ground still so you need to think about what your grounding and the possible current loops you may be setting up.

The idea of a high power and value resistor (A MEG or more) to ground will prevent the drifting up of the isolated side but again if you connect the GFCB it negates that resistor as it will be shorting it out

the secondary side had a ground connection through the GFCB

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/gfi3.html#c1

The ground is where the inbalance occurs, ie if the phase an neutral currents are different then it must be going to earth (or another phase as some have discovered).

http://www.siemens.com/download?BTLV_40730

The only ground I see is the ground wire on the load side, not the GFCB.

Mark

BTW we don't switch neutral on this side of the world ... that just leads to big problems.

I agree but it was shown in this image which is why i mention it

also both sides of the transformer are grounded, it is  defeating the purpose of the isolating transformer, as it is a 1:1 transformer  there is still 110V or 240V AC depending on region on the output and now you can still shock yourself to ground or a grounded part of the equipment under test.

Without the transformer being grounded, the GFCB will not work which is why I say it is not needed in a truly isolate situation. GFCB's are for personal protection and as such needs the ground to provide an alternate route back to the transformer to cause the in-balance. If there was no alternate return path, there could never be an in-balance. there could be over 1000V on the isolated side of the transformer and a person could technically hold on to either side (ONE AT A TIME) and never get a shock (I DON'T RECOMMEND TESTING THAT THEORY THOUGH AS A MISTAKE WILL BE FATAL) but if either side is grounded as in this diagram then all bets are off without the GFCB to provide a safety valve.

I agree with you all that having two GFCB's is pointless and that one should go with the single 5mA version

But I have yet to see how the transformer will provide protection if both sides are grounded , that's not what there used for.

Peter

Yes the sketch that Mike provided does 'appear' to have two grounds ...but does it ground the transformer?

The first is a monitored resistor and he did not include the value, which if it was 25M ohms would not be much of a ground.

I don't know why you would have this, since you could just use a meter between ground and one leg of the transformer ... if it goes above zero then something is not 100%.

The second ground only connects to the GFCB, and every GFCB drawing I've seen with an earth also had two 3 pin outlets.

The earth was connected to the 3rd pin on the sockets.

Without the transformer being grounded, the GFCB will not work

You continue to say this and I think the problem is with the name.

The examples of GFCB I showed above from Siemens (a very well respected company) have no earth and operate the same as our RCD.

Yes in order for the inbalance to occur there must be leakage, and unless it leaks to a higher voltage, then by elimination it must be ground ...

The GFCB doesn't need a ground to work, it just needs the inbalance.

I do admit to seeing on TV an example where they also supply surge protection which requires the earth ...

Mark

The drawing with the monitored ground resistor and the GFCI was someone elses that I posted just for ideas.  It was NOT my intention to use the resistor.  I did think that I could get away with the GFCI though because my understanding was that the GFCI would just detect a current imbalance between either of the legs and if there was an imbalance it would break the circuit.  I went looking for a schematic of the internals of a GFCI and came up with this:

The earth ground appears to be carried through, but there are no connections between the GFCI and ground.  So, Peter, are you telling me that the GFCI will not work unless it is connected to a Neutral that is grounded as would be the case in our normal home power distribution in the US?  So, are you saying that if the P and N were the secondary leads of my isolation transformer and neither of them were grounded that the GFCI would not work?  I think I can't wait for my isolation transformer to arrive so that I can test this.

There are a few earthing systems applicable to the UK. A requirement during installation is to determine the type of supply you have (or will be receiving), as this can affect the protective devices required.

More information can be found here: EARTHING: YOUR QUESTIONS ANSWERED - IET Electrical

As for the Isolation transformer itself. The following has been taken from a document called Safety in electrical testing at work: http://www.hse.gov.uk/pubns/indg354.pdf

• Where an isolating transformer is used for the supply to the equipment under test, it should comply with BS 61558 and a separate transformer used at every test bench. If this is not reasonably practicable, the same isolating transformer may be used for supplies to alternate benches, provided the risk of referencing this supply to earth at any bench is properly controlled and the transformer does not then have an unacceptably high leakage current;
• The supply from the isolating transformer should be provided from a single socket outlet and clearly marked ‘only for use for making live equipment under test’. No fixed wiring should be connected to the earth terminal of the outlet socket. The face plate of the socket should be made of insulating material. There must be no unnecessarily exposed live parts on equipment under test;
• When the equipment under test is Class I, any pre-existing earth fault must be detected and corrected before making the equipment live. In the case of the supply from an isolating transformer, failure to do this will mean that there may be a hazardous shock risk, if there is a simultaneous contact between the enclosure of the equipment and one or both poles of the isolated test supply;
• The integrity of the circuit protective conductor (earth) of all portable/ transportable Class I equipment must be re-tested after all test-bench work has been completed, to make sure there are no earth faults before the equipment is used again on a normal mains supply.

It should be clearly understood, however, that an isolating transformer cannot prevent an electric shock if someone makes contact simultaneously with both conductors of the supply on the secondary side of the transformer, unless the output voltage is below 50 V ac (120 V dc) in dry conditions and 16 V ac (35 V dc) in wet conditions.

Should I worry?