workbench safety: mains isolation

[RayJones]

If you make sure you clip your scope ground to neutral you should be ok ...

Is that really what you meant to say?

Ground is not the same thing as neutral. If, by some freak failure, you loose your neutral connection and you have your scope ground tied to neutral you'll have all of the AC current coursing through ground. Not a good thing.

Also even in the best case where your neutral is intact, if you were using a RCD I think you would likely cause it to trip .  The only time it's really OK to consider ground = neutral is generally at the service entrance where all your bonding points are tied together.

Yes I agree that grounding the neutral beyond a RCD is increasing the likelyhood to trip it, depending upon whatever else is hanging off the RCD.

Yes, our domestic arrangement in Australia also is that Earth and Neutral are tied together at the switchboard's main entry point, and is known as Multiple Earth Neutral - MEN.

The RCD then monitors the ideally balanced current in Active and Neutral from beyond that point. ie Neutral != Earth after the RCD.
There are actually two neutral bars - before and after the RCD, and one just happens to be earthed.

If you are using an isolation transformer, by all means you can connect the probe ground to neutral, but solely with an RCD - no.

[tweek]

I was told once in Italy that if you got a shock off a tap it was because someone was using the earth as neutral to evade logging their consumption on the meter, To be honest I don't know why we don't earth the neutral in buildings ? that way we know that neutral is neutral

I don't know how they do it elsewhere but as a kid I used to do residential electric here in the US (father is a retired electrician) and I'm pretty knowledgeable on how it's done here.  Where the supply comes in, usually at the main breaker panel, you bind the neutral, earth ground and water ground (water pipes, including jumping out the water meter and heater) to a single common bus bar.  At the main panel all grounds and neutrals come back to that bus bar.  In the context of the home electric system, that IS your zero volt reference.  As you branch from the pain panel, current in the neutral will cause a voltage drop which makes it at a higher potential than ground and therefore no longer acceptable to be taken as he same as ground. which is why they are always run out separately, and why neutral is insulated whereas ground doesn't have to be.  So at least here they do in fact bind neutral and earth at the service entrance.  After that all bets are off.

Phil

[Simon]

as far as i know in the UK neutral is only grouded at the substation but i may be wrong

[RayJones]

I believe in the older days the earth was provided by available copper plumbing.

These days a massive copper grounding stake needs to be driven into the ground adjacent to the switchboard.
It's over 1.2m long in the ground.

[Zero999]

Safety regulations here demand both DUT and measuring equipment connected to isolation transformers, if you go anywhere near mains power.

Why would you need to connect the measuring equipment to an isolation transformer?

Modern 'scopes are double insulated which means the outputs are isolated from earth anyway so it gains you nothing.

One thing to note is although that devices connected to an isolation transformer don't need to have an earth connection, their metal cases should all be bonded together so they can't float at different voltages.

To be honest I don't know why we don't earth the neutral in buildings ? that way we know that neutral is neutral

If the neutral wasn't connected to earth the mains could float at any voltage. Here in the UK we have a three phase system so if one live were to become connected to earth, the other lives would float at 400V. Worse still if the mains wasn't earthed and someone connected a high voltage autotransformer which developed an earth fault, the whole mains would float at a dangerously high voltage causing the insulation to fail, smoke and fire. From memory the electrical regulations state that an autotransformer must not be connected to an isolation transformer.

as far as i know in the UK neutral is only grouded at the substation but i may be wrong

It depends on the earthing system used. Where I live, the neutral and earth share a cable until they enter the property where they're separated. The cable is coaxial with the live in the centre and the earth/neutral on the outside. This kind of system is known as TN-C-S.
http://en.wikipedia.org/wiki/Earthing_system

A workbench with an isolation transformer is an example of an IT system.

[tweek]

While a little off topic, here is the electrical diagram of my new workbench. Some will certainly say that it is over the top, but I already own everything in the diagram, so why not use it?

My isolation transformer is a stand alone, so it's not included in the drawing. I always use it for HV DUT, but I never have for my test equipment. If one side is isolated, never saw the necessity to isolate the other.

Wow, I'm just going to be jealous in advance because I'm pretty sure my setup will not be that nice.

[RayJones]

Modern 'scopes are double insulated which means the outputs are isolated from earth anyway so it gains you nothing.

My Rigol DS1052E is definitely grounded at the BNC inputs, I just checked it.
So unfortunately you cannot make such wide sweeping statements.

[JohnS_AZ]

The BNCs on my old HP scope are tied to chassis and electrical ground as well.

[JohnS_AZ]

Wow, I'm just going to be jealous in advance because I'm pretty sure my setup will not be that nice.

There's really nothing special there. It just takes into account 30 years of bitches about other benches. Really, take away the ESD monitor and the EMI filter (which no hobbiest really needs anyway) and it's not even all that expensive.

[Zero999]

The BNCs on my old HP scope are tied to chassis and electrical ground as well.

My old Tektronics and Cossor 'scope but I thought modern plastic cased 'scopes were class 2? May be not as I've not got one myself but I've used one before which was.

Modern 'scopes are double insulated which means the outputs are isolated from earth anyway so it gains you nothing.

My Rigol DS1052E is definitely grounded at the BNC inputs, I just checked it.
So unfortunately you cannot make such wide sweeping statements.

All right may be I'm wrong about that but how would an isolation transformer change that?

Unless the 'scope chassis isn't bonded to the chassis of the device under test there's a risk of a high voltage between them.

The 'scope is normally run off an isolation transformer or SMPS anyway, all my old 'scopes certainly and I've seen pictures of newer 'scopes which suggest that to be the case.

[RayJones]

The isolation transformer assists greatly with a SMPS.

The input circuit is a full wave bridge - active and neutral are the inputs.

If you apply your grounded CRO probe to the -ve of the rectifier, you have just shorted half the rectifier out - not good.

When you place an isolation transformer on the input, neutral is no longer bonded to earth and you can place you CRO's ground anywhere safely.

Note that this should however only be done with the power off, and the bulk capacitors discharged.

[Zero999]

The isolation transformer assists greatly with a SMPS.

The input circuit is a full wave bridge - active and neutral are the inputs.

If you apply your grounded CRO probe to the -ve of the rectifier, you have just shorted half the rectifier out - not good.

When you place an isolation transformer on the input, neutral is no longer bonded to earth and you can place you CRO's ground anywhere safely.

I know that, in fact I suggested it, a few posts back.

You've misunderstood me, I meant using an isolation transformer to power your test equipment is a waste of time.

Note that this should however only be done with the power off, and the bulk capacitors discharged.

Not sure what you mean, of if the power supply is switched of then the neutral isn't connected to the capacitor or anything else.

[tweek]

The isolation transformer assists greatly with a SMPS.

The input circuit is a full wave bridge - active and neutral are the inputs.

If you apply your grounded CRO probe to the -ve of the rectifier, you have just shorted half the rectifier out - not good.

When you place an isolation transformer on the input, neutral is no longer bonded to earth and you can place you CRO's ground anywhere safely.

I know that, in fact I suggested it, a few posts back.

You've misunderstood me, I meant using an isolation transformer to power your test equipment is a waste of time.

Note that this should however only be done with the power off, and the bulk capacitors discharged.

Not sure what you mean, of if the power supply is switched of then the neutral isn't connected to the capacitor or anything else.

I think the intention is to make sure that the SMPS is not energized when test leads are connected.  If the caps aren't discharged, then there is still a ton of energy in the box.  Not an isolation issue just a general safety issue.

[RayJones]

I have never advocated using an isolation transformer for test equipment. I also agree and personally think that is not the sane thing to do.

I was referring to moving the ground clip about in your SMPS. Yes in theory you could move it whilst powered, but I wouldn't.
There is a shit load of joules in that cap just waiting to bite.

[RayJones]

I'll try and reword my reasoning.

Are you likely to be shocked by your CRO?
NO - you'd smash it up and throw it away if that was ever likely to happen, so why isolate it?

Are you likely to be zapped by a mains drive SMPS.
BY JINGOS YES - damn dangerous devices.
If you float the whole lot with an isolation transformer that nasty return path to ground now has an air gap at the transformer.
Grab the active after the tranny - no shock.
It however still does not protect from direct connection to active AND neutral, or any other double touches like the aforementioned bulk cap.

You still must have your wits about you, but the primary benefit is be able to probe with grounded test devices and not blow the living bejesus out of something that may have already been OK.

[Psi]

While a little off topic, here is the electrical diagram of my new workbench. Some will certainly say that it is over the top, but I already own everything in the diagram, so why not use it?

My isolation transformer is a stand alone, so it's not included in the drawing. I always use it for HV DUT, but I never have for my test equipment. If one side is isolated, never saw the necessity to isolate the other.

That's a very nice setup.

Do you use a large rocker switch as Psi showed in his picture

Yeah, i only used the rocker switch because i already had it and the industrial emergency stop buttons were expensive.
Ya should probably use an emergency stop button if you can.
However they're mainly for areas where many people are working so one person can hit the button quickly if they see something bad happening to someone else.
When it's just you working at your hobby desk your hardly going to be able to press any button while getting a shock. So when there's just one person the switch is more there to shut off power when you see smoke, and by then the damage to your circuit is normally already done. Cutting the power superquick with a emergency stop button is unlikely to save your circuit.

Of course with an emergency stop button you do get the advantage of a switch design with safety in mind. You can be sure that if the switch fails it will fail to a safe state

[tweek]

Do you use a large rocker switch as Psi showed in his picture

Yeah, i only used the rocker switch because i already had it and the industrial emergency stop buttons were expensive.
Ya should probably use an emergency stop button if you can.
However they're mainly for areas where many people are working so one person can hit the button quickly if they see someone bad happening to someone else.
When it's just you working at your hobby desk your hardly going to be able to press any button while getting a shock. So when there's just one person the switch is more there to shut off power when you see smoke, and by then the damage to your circuit is normally already done. Cutting the power superquick with a emergency stop button is unlikely to save your circuit.

Of course with an emergency stop button you do get the advantage of a switch design with safety in mind. You can be sure that if the switch fails it will fail to a safe state

I've also been kicking around heading to a marine supply store and getting one of those cut-off switches you find in boats where you can hook something to yourself and it kills the engine if you fall out of the boat.  I'm not sure that solves any more problems either, but it would look cool.

I've got a couple more weeks before I can actually act on most of this so for now I will just have to satisfy myself by thinking of stuff, and ordering that transformer.

[JohnS_AZ]

[ Red Mushroom Emergency Stop Push Button Switch ]

http://www.sourcingmap.com/red-mushroom-emergency-stop-push-button-switch-p-89803.html?utm_source=google&utm_medium=froogle&utm_campaign=usfroogle

8-)

[Chasm]

The main advantage of an Emergency Stop Button on a workbench is that it can be operated really easily. Even if you are working alone.

It is often mounted in a position that can not be obstructed, also an important feature.

[Psi]

[ Red Mushroom Emergency Stop Push Button Switch ]

http://www.sourcingmap.com/red-mushroom-emergency-stop-push-button-switch-p-89803.html?utm_source=google&utm_medium=froogle&utm_campaign=usfroogle

8-)

im not sure, getting a cheap emergency stop button kinda defeats the purpose of having one.
Would want to check the switch quality definitly.

[RayJones]

If you go emergency stop, you should go the latching relay version - they probably have a fancier name than that though!

That way you can easily have multiple stop locations, and should the power be lost to the bench, you won't get any surprises when it returns.

[JohnS_AZ]

im not sure, getting a cheap emergency stop button kinda defeats the purpose of having one.
Would want to check the switch quality definitly.

Well, NO switches fail now and then. NC switches almost never do. However, here ya go ...

http://www.omega.com/pptst/OMPBD7_mush2.html

[Psi]

im not sure, getting a cheap emergency stop button kinda defeats the purpose of having one.
Would want to check the switch quality definitly.

Well, NO switches fail now and then. NC switches almost never do. However, here ya go ...

http://www.omega.com/pptst/OMPBD7_mush2.html

hehe yeah, thats about the price they were when i was designing the power box for my desk.

[alm]

Safety regulations here demand both DUT and measuring equipment connected to isolation transformers, if you go anywhere near mains power.

Why would you need to connect the measuring equipment to an isolation transformer?

Modern 'scopes are double insulated which means the outputs are isolated from earth anyway so it gains you nothing.

One thing to note is although that devices connected to an isolation transformer don't need to have an earth connection, their metal cases should all be bonded together so they can't float at different voltages.

99% of the scopes I've seen are IEC protection class I (grounded, not double isolated), double isolated would be class II (without ground connection). Class I equipment relies on a good ground connection to protect the user (anything connected to ground is by definition at a safe potential). Most scope vendors strongly recommend against floating scopes, it's unsafe. I guess you could use an isolation transformer as long as the ground connection is left intact, but what's the point, the scope already has a transformer? The only exception would be something like a Tek A6901 ground isolation monitor, which continuously monitors the potential between safety ground and scope ground, and will protect the user if it reaches dangerous levels (above 30V or so) by turning off the scope and grounding it.

Just use the proper tools: isolated scopes (which is not just a battery powered scope) or differential probes. Messing around with safety ground and isolation transformer without knowing what you're doing will increase risks, not decrease them.

[Chasm]

Just use the proper tools: isolated scopes (which is not just a battery powered scope) or differential probes. Messing around with safety ground and isolation transformer without knowing what you're doing will increase risks, not decrease them.

As so often it is about knowing what you are doing.

Isolation transformers, HV probes, differential probes, isolated scopes and many other things are tools designed for very specific purposes. As long as you use them as intended they are pretty safe.

Once you do something stupid they will do their best to kill you. Simple as that.

The problem is usually starts with my first sentence, knowing what you are doing. That is usually pretty hard if you were not trained in the specific aspects.

Take the isolation transformer.
It will protect you against one fault, touching a hot wire. It will not protect you against a second fault, aka touching another wire. That is pretty obvious. It will also circumvent the RCD device you may be using on your bench. As far as it is concerned the current that goes into the isolation transformer comes back via neutral and it will not trigger.

Really, knowing the pitfalls of your tools is essential.