Step Down Transformer (what's my best option?)

[Dr.EM]

It's easy to remove, the transformer connections are made by screw terminal blocks, just joined the earth to the centre tap of the secondary. I don't know why the scope is grounded, it's second hand from eBay and a very cheap Farnell model (circa 1984). We used them at college and I remember problems there, a diode in some guys project kept blowing when they connected it to the scope for readings

 

[Nigel Goodwin]

It's easy to remove, the transformer connections are made by screw terminal blocks, just joined the earth to the centre tap of the secondary. I don't know why the scope is grounded, it's second hand from eBay and a very cheap Farnell model (circa 1984). We used them at college and I remember problems there, a diode in some guys project kept blowing when they connected it to the scope for readings

Scopes come earthed - which I find particularly dangerous, as do most of the service trade - but as a class 1 device, that's what the regulations require.

 

[Hero999]

Don't attempt this! - Europe doesn't use the strange American dual-voltage system with two lives (a two phase system).

You mean a split phase supply. A two phase supply is when you have to phases 90 degrees out from each other not a single phase supply from a centre trapped transformer.

Scopes come earthed - which I find particularly dangerous

I disagree, unearth your scope then measure the voltgae on a filter capacitor in a switched mode PSU. Be careful though as the scope's case will be floating at HV! Just don't touch it!

 

[Nigel Goodwin]

You mean a split phase supply. A two phase supply is when you have to phases 90 degrees out from each other not a single phase supply from a centre trapped transformer.

Possibly so?, I've no idea how it's implemented - it just seems a really bizare idea? - presumably a desparate measure to compensate for choosing too low a mains voltage?.

I disagree, unearth your scope then measure the voltgae on a filter capacitor in a switched mode PSU. Be careful though as the scope's case will be floating at HV! Just don't touch it!

I do it ALL the time - do it with an earthed scope and there's a massive bang (if you're lucky!), if you're not lucky you get a possibly fatal shock directly across the heart. I consider that FAR more dangerous than the scope casing floating at 115V (half mains), when you know perfectly well that's what it's doing!.

As usual, I'll mention that you shouldn't do it unless you fully understand the reasons why! - but you shouldn't be using a scope in the first place if you don't!. It's a common place technique in the service trade, including multi-national companies training rooms!.

 

[Dr.EM]

Makes me glad I only use it with low voltages and a short circuit protected power supply. I don't fully understand the earthing business here, so I best keep it that way

 

[Hero999]

A LOT depends on exactly what you use the transformer for, and how the unit is cased - if you have a metal case it's probably a good idea to earth the case, this doesn't mean the secondary of the transformer needs earthing - it's a completely different scenario.

Wikipedia has a good section on earthing systems (I know it's three phase but it's still relevent to single phase).

https://en.wikipedia.org/wiki/Earthing_system

In my oppinion a IT earthing system is fine just for powering one appliance off, but I certainly wouldn't recommend it for power distribution i.e. powering your home/lab from a non-earthed-bonded isolation transformer.

Possibly so?, I've no idea how it's implemented - it just seems a really bizare idea? - presumably a desparate measure to compensate for choosing too low a mains voltage?.

That's true, the US buildings are fed from 120V-0-120V, 240V is used for water heaters and 110V for everything else. Those building site transfromers, also have a split phase output, 55V-0-55V.

https://en.wikipedia.org/wiki/Split_phase

I do it ALL the time - do it with an earthed scope and there's a massive bang (if you're lucky!),

Exactly the fuse will blow, thus protecting you!

if you're not lucky you get a possibly fatal shock directly across the heart.

Only if you have a poor quality earth or power connection or you've replaced that fuse with a nail.

I consider that FAR more dangerous than the scope casing floating at 115V (half mains), when you know perfectly well that's what it's doing!.

You're wrong, the case will float at full mains potential!

As usual, I'll mention that you shouldn't do it unless you fully understand the reasons why! - but you shouldn't be using a scope in the first place if you don't!. It's a common place technique in the service trade, including multi-national companies training rooms!.

Common practice? Not in reputable companies it isn't and if you did it where I work you'd get sacked pretty quickly!

The only safe way to measure mains connected voltages with an oscilloscope is to power the equipment you're testing from an isolation transformer. This is how it's done in real industry and it's the only safe way of doing it.

 

[Nigel Goodwin]

Exactly the fuse will blow, thus protecting you!

If you touch the scope ground directly to a live chassis it will, but NOT if you have the scope lead in one hand, and the chassis in the other!.

Only if you have a poor quality earth or power connection or you've replaced that fuse with a nail.

Your body isn't going to pass enough current, hand to hand, to blow the fuse in the plug - so the plug fuse is no protection at all for your safety - and is solely intended to blow in the case of damage to the mains lead (the unit itself being 'protected' by an internal fuse).

You're wrong, the case will float at full mains potential!

Not in my experience, nor from theory either, the bridge rectifer means the 'chassis' of the primary of a SMPSU is at half mains potential, it's not connected directly to live, and it's not connected directly to neutral. If you go back a LONG time, to old valve sets, they commonly used a fully live chassis and a half wave rectifier. If you plugged them in one way round the chassis was connected directly to neutral, the other way directly to live.

Common practice? Not in reputable companies it isn't and if you did it where I work you'd get sacked pretty quickly!

You obviously live a very sheltered life!.

 

[Dean Huster]

Nigel, I'd assume that "strange" in the UK equates to "different" in the U.S. When you live with a system, you're comfortable with it. I find it interesting that every household appliance in the UK is 230v while you consider 230v to be dangerous for the carpenters because "builders are completely brain dead where electricity is concerned." Since 230v is OK in a living room (or worse yet, a kitchen or bathroom), does that mean that builders are your only "brain dead" residents? I'd think that if they're pretty smart otherwise but "brain dead" around electricity, that your average accountant, housewife or office worker would really be in trouble in comparison. Across the pond here, we pretty much level the playing field and just use 120v for the household appliances for safety reasons overall, and let our 2-phase availability of 240v operate the permanently-wired high-power loads. We have 3-phase, too, each house supplied by tapping from only one phase, which is then split by a center-tapped transformer to the bi-phase 120/240v system.

Over here, our builders aren't provided with 60vac tools, but it would appear that we force them use those confounded battery-operated ones. Most builders world-over have more trouble with injury from saw cuts, drill stabs, knife cuts, etc. than they do from electrical shock. They mostly need Band-Aids, not CPR!

And regarding the transformers, I'd suppose that most can handle the difference in mains frequency. But if you're using the transformer at near its peak load capability, the 50 Hz may cause it to saturate out at lower load demands than would the 60 Hz current. Most better industrial stuff is rated as 50/60 Hz.

Dean

 

[Hero999]

If you touch the scope ground directly to a live chassis it will, but NOT if you have the scope lead in one hand, and the chassis in the other!.

Now why would you do that?
And I don't see how having the case earthed can make that dangerous.

Your body isn't going to pass enough current, hand to hand, to blow the fuse in the plug - so the plug fuse is no protection at all for your safety - and is solely intended to blow in the case of damage to the mains lead (the unit itself being 'protected' by an internal fuse).

If the scope is earthed it won't be you that's passing the current, it'll be the live and earth conductors, this will blow the either the internal and/or the external fuse in the plug.

Not in my experience, nor from theory either, the bridge rectifier means the 'chassis' of the primary of a SMPSU is at half mains potential, it's not connected directly to live, and it's not connected directly to neutral.

Show me a schematic then, all the units I've seen have the bridge rectifier connected directly across the live and neutral. From earth ground the + rail appears as a stream of +330V pulses and the - rail appears as a stream of -330V pulses. I don't know about you but I would rather not have a big metal box floating at theses voltages, I would much prefer to buy an isolation transformer.

If you go back a LONG time, to old valve sets, they commonly used a fully live chassis and a half wave rectifier. If you plugged them in one way round the chassis was connected directly to neutral, the other way directly to live

I've heard about this in the US but not here in the UK.

You obviously live a very sheltered life!.

Yes, I'm glad I've been sheltered from the cowboy engineers who use unearthed scopes and are too cheap to go out and buy a £50 isolation transformer.

 

[Nigel Goodwin]

Nigel, I'd assume that "strange" in the UK equates to "different" in the U.S. When you live with a system, you're comfortable with it. I find it interesting that every household appliance in the UK is 230v while you consider 230v to be dangerous for the carpenters because "builders are completely brain dead where electricity is concerned." Since 230v is OK in a living room (or worse yet, a kitchen or bathroom), does that mean that builders are your only "brain dead" residents? I'd think that if they're pretty smart otherwise but "brain dead" around electricity, that your average accountant, housewife or office worker would really be in trouble in comparison. Across the pond here, we pretty much level the playing field and just use 120v for the household appliances for safety reasons overall, and let our 2-phase availability of 240v operate the permanently-wired high-power loads. We have 3-phase, too, each house supplied by tapping from only one phase, which is then split by a center-tapped transformer to the bi-phase 120/240v system.

As I understand it, people still get killed by your 120V mains, probably as high a percentage as our 230/240V mains? - the mains voltage doesn't seem to make as much difference as you might think.

I do seem to remember reading somewhere that certain parts of the States (some farms or something?) actually use (or did use) 60V mains - and people have been killed by that as well?.

To be honest, you have to work really hard for 240V to kill you, or be particularly unlucky!

Normal houses in the UK only have a single phase supply, with three phase supplies to industrial premises - in fact at work, where I am now, we have three phase supplies to both the shop and the service department. The shop one is used to feed a small goods lift/hoist, which has a three phase motor, and various parts of the building are fed from different phases (you can tell when a phase goes down). The workshop has a three phase supply going in, but only uses one of them - but it was installed when the workshop was built, for possible future use.

Although the shop and workshop are joined together, interestingly they are powered from completely different directions - so it's possible for one to have power and not the other!.

 

[Hero999]

Nigel, I'd assume that "strange" in the UK equates to "different" in the U.S. When you live with a system, you're comfortable with it. I find it interesting that every household appliance in the UK is 230v while you consider 230v to be dangerous for the carpenters because "builders are completely brain dead where electricity is concerned." Since 230v is OK in a living room (or worse yet, a kitchen or bathroom), does that mean that builders are your only "brain dead" residents? I'd think that if they're pretty smart otherwise but "brain dead" around electricity, that your average accountant, housewife or office worker would really be in trouble in comparison.

There a more water hazards on a building site and I assume they don't want to use an RCD because they don't want the inconvenience of it tripping every five minutes.

Across the pond here, we pretty much level the playing field and just use 120v for the household appliances for safety reasons overall, and let our 2-phase availability of 240v operate the permanently-wired high-power loads. We have 3-phase, too, each house supplied by tapping from only one phase, which is then split by a centre-tapped transformer to the bi-phase 120/240v system.

It isn't 2-phase or bi-phase it's split-phase!

that we force them use those confounded battery-operated ones. Most builders world-over have more trouble with injury from saw cuts, drill stabs, knife cuts, etc. than they do from electrical shock. They mostly need Band-Aids, not CPR!

Fair point, let's not forget falling off ladders and tripping over on things.

And regarding the transformers, I'd suppose that most can handle the difference in mains frequency. But if you're using the transformer at near its peak load capability, the 50 Hz may cause it to saturate out at lower load demands than would the 60 Hz current. Most better industrial stuff is rated as 50/60 Hz.

I agree, if you use a 60Hz transformer on 50Hz, over-rate it by 16.7% in other words use it at no more than 83.3% of the current listed on the label.

 

[audioguru]

Behaviour that is caused by being brain-dead:
Electrocution, saw cuts, drill stabs, knife cuts, falling off ladders and tripping over on things.

 

[Dean Huster]

As I understand it, people still get killed by your 120V mains, probably as high a percentage as our 230/240V mains? - the mains voltage doesn't seem to make as much difference as you might think.

I do seem to remember reading somewhere that certain parts of the States (some farms or something?) actually use (or did use) 60V mains - and people have been killed by that as well?.

Here in the U.S., folks tend to get zapped about equally. It's hard to really come up with reliable stats on that (and stats can be and usually are twisted to suit the person quoting them), but if one were to say that as many are killed by 120v as there are by 240v, it would leave out the fact that the average person over here is potentially exposed to 120v daily whenever they mess with line cords or if equipment fails; 240v is confined within "the walls" and powers virtually no visible appliances in a room, save for a window air conditioner in an ancient home. That would be like saying that lion tamers are injured more by lions than they are by kangaroos. Well, yeah, duh!

I think I can say that there has never been anything lower than 100vac in this country, even in the "olden" days. Rural electrification in the 1930s was either though the old REA (Rural Electrification Administration if memory serves me well today) which supplied 110vac. If REA wasn't available, some farmers had batteries charged by either Delco power plants or windchargers that operated at 32vdc.

Nigel, you and I have often butted heads on the earthing issue. I've just never understood why, when I'd say EVERY industrial test equipment manufacturer ALWAYS earths their equipment, you take the side against such a practice. Actually, Tektronix will advise using a differential mode to work on switchers or other "floating" or line-referenced circuits rather than isolating the scope chassis and floating it until it glows.

Of course, I guess if it's glowing, attracting your hair on dry days and making sparking noises, it's warning to not touch it.

Dean

 

[Hero999]

I just don't know why they don't make oscilloscopes with plastic cases. Then it would be perfectly saft to have them unearthed, providing the outer sheiled on the coax connector was insulated of course.

 

[Nigel Goodwin]

Here in the U.S., folks tend to get zapped about equally. It's hard to really come up with reliable stats on that (and stats can be and usually are twisted to suit the person quoting them), but if one were to say that as many are killed by 120v as there are by 240v, it would leave out the fact that the average person over here is potentially exposed to 120v daily whenever they mess with line cords or if equipment fails; 240v is confined within "the walls" and powers virtually no visible appliances in a room, save for a window air conditioner in an ancient home. That would be like saying that lion tamers are injured more by lions than they are by kangaroos. Well, yeah, duh!

Sorry, I think we are at cross purposes?, I was referring to 120V in the USA and 230V in the UK/Europe - not considering 240V in the USA at all, because (as you say) it's not generally accessible to the public.

But, from your comments, it's pretty obvious that having 120V mains doesn't stop people getting killed?.

I think I can say that there has never been anything lower than 100vac in this country, even in the "olden" days. Rural electrification in the 1930s was either though the old REA (Rural Electrification Administration if memory serves me well today) which supplied 110vac. If REA wasn't available, some farmers had batteries charged by either Delco power plants or windchargers that operated at 32vdc.

I just seem to vaguely remember reading something about it, a LONG time ago, in an article about electrical safety - comparing 120V to 240V.

Nigel, you and I have often butted heads on the earthing issue. I've just never understood why, when I'd say EVERY industrial test equipment manufacturer ALWAYS earths their equipment, you take the side against such a practice. Actually, Tektronix will advise using a differential mode to work on switchers or other "floating" or line-referenced circuits rather than isolating the scope chassis and floating it until it glows.

There are essentially two possibilities - earthing and not earthing, I would consider an earth-free environment a LOT safer than an earthed one, because it removes completely the need for anything to be earthed, and prevents the most shock mode (live to ground).

To get a shock you need a path between live and neutral, the only reason you can get a shock to ground is because the neutral is earthed at the substation. Operating the equipment via an isolating transformer simply breaks that neutral connection to ground, providing you mains voltage on two wires - neither of which is live or neutral.

Your scenario:

Earthed scope, live chassis, hand on each - electrocution.

My scenario:

Non-earthed scope, live chassis, hand on each - no problem.

Which of those two is safer? - bearing in mind this is a situation I'm probably involved in on a daily basis!.

Presumably your point though would be:

Non-earthed scope, live chassis, hand on scope and hand on 'convenient' earth point. This can't happen, because you don't have any 'convenient' earth points to do it with.

As I've said before, it's not just MY opinion, it's commonplace throughout the service world, and even in international manufacturers training rooms.

But as I've also always said - DON'T DO THIS UNLESS YOU KNOW EXACTLY WHY YOU'RE DOING IT, AND WHY IT CAN BE SAFER.

Incidently, all my benches at work are fed from isolation transformers, and the workshop is also effectively 'earth free' - so the scope being 'live' doesn't really apply anyway, unless you happen to use a non-isolated socket?. But I wouldn't consider earthing the scope - it's a danger point, NOT a safety one.

BTW, even though all benches are isolated, I ALWAYS treat them with the respect they would deserve if they were not!.

At home I don't have isolation transformers, and certainly wouldn't consider earthing a scope.

 

[Hero999]

I've thought about this a lot, but here's what Wikipedia has to say about the IT system (I know I've posted the link before but I don't see any harm in quoting the relevent parts from it).

https://en.wikipedia.org/wiki/Earthing_system

**broken link removed**

In IT systems, a single insulation fault is unlikely to cause dangerous currents to flow through a human body in contact with earth, because no low-impedance circuit exists for such a current to flow. However, a first insulation fault can effectively turn an IT system into a TN system, and then a second insulation fault can lead to dangerous body currents. Worse, in a multi-phase system if one of the lives made contact with earth it would cause the other phase cores to rise to the phase-phase voltage relative to earth rather than the phase-neutral voltage. IT systems also experience larger transient overvoltages than other systems.

In IT and TN-C networks, residual current devices are far less likely to detect an insulation fault. In a TN-C system they would also be very vulnerable to unwanted triggering from contact between earths of circuits on different RCDs or with real ground thus making their use impractical. Also RCDs usually isolate the neutral core which is dangerous in a TN-C system.

Laboratory rooms, medical facilities, construction sites, repair workshops, and other environments where there is an increased risk of insulation faults often use an IT earthing arrangement supplied from an isolation transformer. To mitigate the two fault issues with IT systems the isolation transformers should only supply a small number of loads each and/or should be protected with special monitoring gear (generally only medical IT systems are done with such gear because of the cost).

I really can see your point about isolating the power supply from earth but (as listed above it does have its disadvantages.

Here's my suggestion.

Both conductors are connected to 0V by high impedances.

The voltage at point X is monitored with respect to earth, if it's greater than 0V by more than a few hundered mV then the circuit breaker will be tripped to isolate the circuit.

The main advantage this system has to the usual RCD is that normal power supply filters should cause it to trip because they contain equal capacitances to earth from each conductor so it should ballance.

 

[Dean Huster]

It isn't 2-phase or bi-phase it's split-phase!

0° phase difference is the same phase; anything else is "out-of-phase", but there would be two phases, the reference phase and the "out-of-phase" signal. Two signals 180° out-of-phase are normally referred to as "split-phase", but usually I think is confined to its use as the output from a phase-splitter feeding a Class B output amp. Still, split-phase or not, it's two distinct phases, 180° difference. The 180° phases are just pretty handy for additive uses, deriving 240v from a pair of 120v secondaries, albiet a single winding, center-tapped.

Semantics.

PLASTIC CASED SCOPES: Yes, they do/did exist as the Tektronix 200-series, small double-insulated, battery-operated scopes. The 211 and 212 were 500KHz BW, single- and double-trace respectively; the 214 was a 212 with bistable storage; the 221 was the 5MHz model, single channel; and the darling of the group was the 213, a single-channel, 1MHz BW scope with a built-in DMM that provided it's display on-screen with DC and AC voltage and current (true RMS) and resistance. You could actually display a resistance measurement vs. time on the screen, say to look at the output of a light-modulated LDR. The 213 wasn't a bad little scope for the year 1976, except for it's $2500 price tag!

NIGEL'S GROUNDS: Yes, if NOTHING were earth-referenced, then you'd not have the "hot spots". But as we know ......
Nigel, you and I will never agree on this subject, and that's OK. It's what makes the world turn, right?

Dean

 

[audioguru]

It's what makes the world turn, right?

For their wiring, if the electrical companies used another wire instead of using the earth as a return, then electricity would be so expensive that nobody would use it.

 

[Nigel Goodwin]

For their wiring, if the electrical companies used another wire instead of using the earth as a return, then electricity would be so expensive that nobody would use it.

Is that what they do in Canada?, as far as I'm aware it's not done like that in the UK - because it's three phase, and the loads are roughly balanced across the three phases (by feeding alternate houses off alternate phases). So distribution to house level uses three lives and a thin neutral, and high voltage distribution uses just the three lives.

 

[Hero999]

0° phase difference is the same phase; anything else is "out-of-phase", but there would be two phases, the reference phase and the "out-of-phase" signal. Two signals 180° out-of-phase are normally referred to as "split-phase", but usually I think is confined to its use as the output from a phase-splitter feeding a Class B output amp. Still, split-phase or not, it's two distinct phases, 180° difference. The 180° phases are just pretty handy for additive uses, deriving 240v from a pair of 120v secondaries, albiet a single winding, center-tapped.

Split phase and two phase are totally different definitions under Wikipedia.
https://en.wikipedia.org/wiki/Split_phase
https://en.wikipedia.org/wiki/Two_phase

PLASTIC CASED SCOPES: Yes, they do/did exist as the Tektronix 200-series, small double-insulated, battery-operated scopes. The 211 and 212 were 500KHz BW, single- and double-trace respectively; the 214 was a 212 with bistable storage; the 221 was the 5MHz model, single channel; and the darling of the group was the 213, a single-channel, 1MHz BW scope with a built-in DMM that provided it's display on-screen with DC and AC voltage and current (true RMS) and resistance. You could actually display a resistance measurement vs. time on the screen, say to look at the output of a light-modulated LDR. The 213 wasn't a bad little scope for the year 1976, except for it's $2500 price tag!

Do they still sell them though?

Maybe I should consider building an oscilloscope isolation amplifier for my next project then!

Nigel, you and I will never agree on this subject, and that's OK. It's what makes the world turn, right?

Me included. Earthing a scope always has made sense to me and the more I read Nigel's arguments against it the more I believe I'm right.