Go ahead,
post your instructions.
List of equipment,
Method of assembly
Method of testing correct assembly of the Isolated Test Bench.
etc.
post your instructions.
List of equipment,
Method of assembly
Method of testing correct assembly of the Isolated Test Bench.
etc.
So it seems that it's a great challenge to build a test bench that is both safe and meets the codes.
It the path we had them in the high power lab, but no-way would they meet the present code.
It the path we had them in the high power lab, but no-way would they meet the present code.
Mortice and tenon softwood frame with aliphatic resin glued stepped dowels thru the tenons for rigidity and a 25mm medite table top, glued and biscuit jointed all round to the top of softwood frame.
Feeling richer, replace softwood with hardwood for the frame and marine ply or hardwood for the worktop.
For a finish, a natural resin marine varnish like Le Tonkinois.
No metal in the equation and wood still holds its own as an excellent electrical insulator in the modern age.
Feeling richer, replace softwood with hardwood for the frame and marine ply or hardwood for the worktop.
For a finish, a natural resin marine varnish like Le Tonkinois.
No metal in the equation and wood still holds its own as an excellent electrical insulator in the modern age.
I have an old door with a sheet of replaceable hardboard as the working surface.
It is stood on 4 Ikea's legs.
It is stood on 4 Ikea's legs.
this is like a guessing game.
people suggest something Andrew gets to knock it down due to not meeting some invisible criteria in his mind.
code? don't build it in a fixed location. use bench test gear and equip thy brain with knowledge / skills for the task at hand. working on primary circuits is a non starter for the general DIY crowd.
why don't you tell us what yer testing and any testing you deem to be potentially unsafe
BTW "isolated" has so many meanings to different ppl like a screen room or perhaps secret testing on military frequencies
people suggest something Andrew gets to knock it down due to not meeting some invisible criteria in his mind.
code? don't build it in a fixed location. use bench test gear and equip thy brain with knowledge / skills for the task at hand. working on primary circuits is a non starter for the general DIY crowd.
why don't you tell us what yer testing and any testing you deem to be potentially unsafe
BTW "isolated" has so many meanings to different ppl like a screen room or perhaps secret testing on military frequencies
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Andrew does it again!
Well, where I work, we use wooden benches and judicious flying earths.
Beyond that, well....is it safe to use anymore?
If a serious suggestion, then a few things should be spec'd.
Isolation voltage?
Reason for live work? Justifications?
More specifically we use phenolic ply tops, metal legs and supports, isolating mount feet. Frame grounded and flying earths for equipment. Interlocked variac controlled AC supply, multi tapped Xformer and guarded terminals.
HSE makes live work interesting, to say the least.
Well, where I work, we use wooden benches and judicious flying earths.
Beyond that, well....is it safe to use anymore?
If a serious suggestion, then a few things should be spec'd.
Isolation voltage?
Reason for live work? Justifications?
More specifically we use phenolic ply tops, metal legs and supports, isolating mount feet. Frame grounded and flying earths for equipment. Interlocked variac controlled AC supply, multi tapped Xformer and guarded terminals.
HSE makes live work interesting, to say the least.
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could you explain "flying earths"?
My voltages are UK mains.
i.e. when I power up a new mains powered amplifier. It has mains at one end and me at the other.
Other Members state that the oscilloscope should never be isolated. I agree.
That leaves the scope and it's probe/s connected to Mains PE.
What do we do to isolate the DUT?
What protections must be fitted? What would be nice to have?
There has been discussion on how to wire up an isolation transformer. But this amateur and the other Member did not arrive at a conclusion.
We needed professional advice.
My voltages are UK mains.
i.e. when I power up a new mains powered amplifier. It has mains at one end and me at the other.
Other Members state that the oscilloscope should never be isolated. I agree.
That leaves the scope and it's probe/s connected to Mains PE.
What do we do to isolate the DUT?
What protections must be fitted? What would be nice to have?
There has been discussion on how to wire up an isolation transformer. But this amateur and the other Member did not arrive at a conclusion.
We needed professional advice.
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Flying earths:
Flying earth leads,all bonded to a common starpoint on a metal table frame, which is then connected to a main earthing busbar. Useful for chassis earths etc when live working.
Commonly we use 0-750 Vac variac controlled. A simple microswitch under a 'letter flap' of insulating board (or a box) covering exposed live ac output terminal binding posts. This is in series with the coil of the supply contactor to prevent supply actuation while terminals are exposed.
As for scope earths, we do isolate scopes with the use of a isolating transformer and extreme care. (often I am measuring impulses and rise times via 1000:1 HT probe at up to 40kV on a portable bench. Since we are interested in relative voltages, scope earth is not at 0V or ground potential, but is considered 'earthy' end of the circuit)
Hence care still has to be exercised. (I have seen sparks jump the bnc on scope inputs when transients over 400V are present, or when transients appear on the real earth)
So in those cases isolation is necessary to prevent destruction.of the scope.
Id expect scope isolation os not really necessary in your case, where voltages will not exceed the typical 400V scope.input limits
As for DUT...well I don't believe an isolating transformer is required for AC supply. Just use the L and N from variac or similar (or even just the mains socket) the mains earth then bonds to the frame and star point for the safety flying earths (croc clips are handy here)
As for scope probe earth and amplifier outputs, I suspect you are looking at differential output, and the scope earth can be affixed to the amp lowside earthy end at the output, probe tip.to the high side. As long as you dont exceed the scope isolation on p-p swing all should be fine.
Its trickier if you want absolute voltage measurement as the mains earth is generally so dirty that I wouldn't think you'd gain anything useful by using it. Use a 10:1 probe and you won't be shunting too much current either. Bnc to banana plug 'straight thru' leads are a source of many nightmares in my limited experience.
EDIT: I got the table material wrong. It is actually melamine faced board, easy to clean and prevent tracking or discharge at mains voltages.
Apologies for the punctuation hiccups, hopefully my short essay is clear and answers your questions.
Pm if you need, I have some bench sch lying around c1940, that are good to use by the electrically competent (and I'm sure you are)
Flying earth leads,all bonded to a common starpoint on a metal table frame, which is then connected to a main earthing busbar. Useful for chassis earths etc when live working.
Commonly we use 0-750 Vac variac controlled. A simple microswitch under a 'letter flap' of insulating board (or a box) covering exposed live ac output terminal binding posts. This is in series with the coil of the supply contactor to prevent supply actuation while terminals are exposed.
As for scope earths, we do isolate scopes with the use of a isolating transformer and extreme care. (often I am measuring impulses and rise times via 1000:1 HT probe at up to 40kV on a portable bench. Since we are interested in relative voltages, scope earth is not at 0V or ground potential, but is considered 'earthy' end of the circuit)
Hence care still has to be exercised. (I have seen sparks jump the bnc on scope inputs when transients over 400V are present, or when transients appear on the real earth)
So in those cases isolation is necessary to prevent destruction.of the scope.
Id expect scope isolation os not really necessary in your case, where voltages will not exceed the typical 400V scope.input limits
As for DUT...well I don't believe an isolating transformer is required for AC supply. Just use the L and N from variac or similar (or even just the mains socket) the mains earth then bonds to the frame and star point for the safety flying earths (croc clips are handy here)
As for scope probe earth and amplifier outputs, I suspect you are looking at differential output, and the scope earth can be affixed to the amp lowside earthy end at the output, probe tip.to the high side. As long as you dont exceed the scope isolation on p-p swing all should be fine.
Its trickier if you want absolute voltage measurement as the mains earth is generally so dirty that I wouldn't think you'd gain anything useful by using it. Use a 10:1 probe and you won't be shunting too much current either. Bnc to banana plug 'straight thru' leads are a source of many nightmares in my limited experience.
EDIT: I got the table material wrong. It is actually melamine faced board, easy to clean and prevent tracking or discharge at mains voltages.
Apologies for the punctuation hiccups, hopefully my short essay is clear and answers your questions.
Pm if you need, I have some bench sch lying around c1940, that are good to use by the electrically competent (and I'm sure you are)
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Additionally, for the more safety conscious you can also include a 'deadman' switch - think guitar pedal footswitch, which must be pressed to close the supply contactor. If you should be unlucky enough to be shocked and knocked off the switch, the supply will be cut off, possibly saving a life.
(ive built such a testing area, albeit for higher voltages, but the dead man switch is an integral safety feature, and considered essential)
EDIT: Again I made an error and missed something in what you wrote.
An isolating transformer may be necessary after all. I'd suspect if using 10:1 or greater probes then you can probably omit it, and use a RCD to detect excess current to ground through the scope probe earth. Otherwise a 1:1 transformer would be sensible. Ive never had issue unless non attenuated probes are used.
As is usual, a picture tells a thousand words.
I'll have a look about and see if I can dig a sch out for you.
(ive built such a testing area, albeit for higher voltages, but the dead man switch is an integral safety feature, and considered essential)
EDIT: Again I made an error and missed something in what you wrote.
An isolating transformer may be necessary after all. I'd suspect if using 10:1 or greater probes then you can probably omit it, and use a RCD to detect excess current to ground through the scope probe earth. Otherwise a 1:1 transformer would be sensible. Ive never had issue unless non attenuated probes are used.
As is usual, a picture tells a thousand words.
I'll have a look about and see if I can dig a sch out for you.
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Following my train of thought (admittedly rather sluggish)...
If the bench is for amplifer testing I cant see a reason why you would need most of these precautions.
How about a simple AC supply, ideally variac controlled which in turn feeds a 'universal' rectification and reservoir bank?
A dead man switch to enable DC out put to the amplifier, via a suitable contactor. On the release of dead man switch, contactor (DPDT) switches out and discharges reservoir caps through a suitable earthing resistor. A second DPDT contactor on the same interlock circuit engages AC supply to the variac control, and breaks that circuit respectively (so you don't end up leaving AC on and DC side discharging resistor in circuit)
Since you aren't really testing with AC (unless there's an AC fed amplifier I'm not aware of 🙂)
If the bench is for amplifer testing I cant see a reason why you would need most of these precautions.
How about a simple AC supply, ideally variac controlled which in turn feeds a 'universal' rectification and reservoir bank?
A dead man switch to enable DC out put to the amplifier, via a suitable contactor. On the release of dead man switch, contactor (DPDT) switches out and discharges reservoir caps through a suitable earthing resistor. A second DPDT contactor on the same interlock circuit engages AC supply to the variac control, and breaks that circuit respectively (so you don't end up leaving AC on and DC side discharging resistor in circuit)
Since you aren't really testing with AC (unless there's an AC fed amplifier I'm not aware of 🙂)
That's quite a few avenues for me to consider and decide which are appropriate for my "low voltage" work (240Vac Mains).
Thanks for the pointers.
Thanks for the pointers.
This is a reply to Infinia in another Thread.
If I am using my lab power supply to power up an experiment/project then I am relying on the isolation provided by the sealed up lab supply.
I see no problem with that arrangement.
I will need to go and check, but I am pretty sure the socket outlets under my bench are supplied via a 25A 30mA RCBO in my distribution board fusebox. There are six RCBO in there for all my power rings around the house. Dedicated power to water heater, oven, central heating and 3phase tools, only have MCBs
If I am powering up a project with mains power into a "normal" mains transformer, I have Live & Neutral on the bench.
That could lead to mistakes and/or injury.
It is for this that I would like something safer, so that if a mistake were made I survive and preferably the project survives. MCB/s and RCD/RCBO on the isolating transformer may be required.
A "nice to have" feature would be the ability to connect the scope probe ground lead to any part of a mains powered project and be able to measure the voltage across a component. I think an isolated supply would allow this, but I may be wrong.
Interference attenuation is a different issue.
Any attenuation would be secondary and after the safety side is sorted.
I could use screened/shielded mains cable into and out of the isolating transformer. I could use a PE connected enclosure around that transformer. I could add on a mains filter, either the canned type, or I have components to add on a second more elaborate if required.
If I am using my lab power supply to power up an experiment/project then I am relying on the isolation provided by the sealed up lab supply.
I see no problem with that arrangement.
I will need to go and check, but I am pretty sure the socket outlets under my bench are supplied via a 25A 30mA RCBO in my distribution board fusebox. There are six RCBO in there for all my power rings around the house. Dedicated power to water heater, oven, central heating and 3phase tools, only have MCBs
If I am powering up a project with mains power into a "normal" mains transformer, I have Live & Neutral on the bench.
That could lead to mistakes and/or injury.
It is for this that I would like something safer, so that if a mistake were made I survive and preferably the project survives. MCB/s and RCD/RCBO on the isolating transformer may be required.
A "nice to have" feature would be the ability to connect the scope probe ground lead to any part of a mains powered project and be able to measure the voltage across a component. I think an isolated supply would allow this, but I may be wrong.
Interference attenuation is a different issue.
Any attenuation would be secondary and after the safety side is sorted.
I could use screened/shielded mains cable into and out of the isolating transformer. I could use a PE connected enclosure around that transformer. I could add on a mains filter, either the canned type, or I have components to add on a second more elaborate if required.
Hi Andrew
I think the safest way to power up a project, is using a variac with a resettable breaker along with a giant sized analog AC amp meter and possibly a digital AC voltmeter. Mount this arrangement together in a sloped metal box or a custom panel underneath the benches upper shelf. Perhaps leave room for an added isolation transformer to another output of this for the rarer occasions you need to probe AC. The analog amp meter is fast acting and should always remain in your peripheral vision next to the AC power on/off switch.
importantly the variac needs to be 'up sized' if you do low line full power testing, a good thing many ppl fail to check!
PS you probably already have this arrangement in some way IDK maybe reorganizing them in a more suitable way can free up space and streamline things a bit.
I think the safest way to power up a project, is using a variac with a resettable breaker along with a giant sized analog AC amp meter and possibly a digital AC voltmeter. Mount this arrangement together in a sloped metal box or a custom panel underneath the benches upper shelf. Perhaps leave room for an added isolation transformer to another output of this for the rarer occasions you need to probe AC. The analog amp meter is fast acting and should always remain in your peripheral vision next to the AC power on/off switch.
importantly the variac needs to be 'up sized' if you do low line full power testing, a good thing many ppl fail to check!
PS you probably already have this arrangement in some way IDK maybe reorganizing them in a more suitable way can free up space and streamline things a bit.
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I use variac controlled500V/A isolating transformer for my DUT on a single socket clearly marked as Isolated Supply. Raw mains for test gear and all raw mains carries an earth. On a wooden bench with a 10mA RCD breaker. That ensures that if it all goes horribly wrong, the maximum shock hazard is limited to 10mA. There is a large red Test button (latching switch) that places a 22K 1W resistor from Live to Earth to test the trip and power down the whole bench. Never let the DUT have a path to earth unless using test gear.
I use this type of test bench as it is a requirement of IEE regulations to provide a safe and controlled place of work in an electrical repairs workshop.
I use this type of test bench as it is a requirement of IEE regulations to provide a safe and controlled place of work in an electrical repairs workshop.
Is that RCD on the supply feed to the isolating transformer, or on the output from the isolating transformer?
It would be nice if you could sketch and post the wiring diagram for the two sets of supplies (isolated or PE protected).
What does this mean?
It would be nice if you could sketch and post the wiring diagram for the two sets of supplies (isolated or PE protected).
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I've been doing this professionally for over 40 years, and in the last few years moved to a new smaller workshop, and had a 'bare canvas' to start from. I built and wired it as I wanted.
Essentially the wiring from scratch is split in two completely separate areas - one is permanent power (for computers, networking, security systems etc. + a couple of sockets in a soak test area for items that need long term testing).
The rest of the wiring is switched by a master switch (actually a cooker switch 😀), you simply knock the switch off when you leave, and everything you want off goes off.
I have three benches, each with 1KW isolation transformers, with sockets on the front of the benches switched by a double pole fused switch. So while working on an item you can switch the power off by using that switch.
On the sides of the benches I have four way extension sockets mounted, fed with live (non-isolated and non-switched) power, where you plug test equipment in - you don't want the scope and soldering iron going off with the item you're working on. TV aerial sockets are also isolated - BUT satellite ones aren't (they can't be) and neither are network and phone sockets.
As far as scopes go, as is common in the service trade, I use non-earthed scopes - I consider an earthed scope too dangerous to use in what is essentially an earth free environment (but I wouldn't suggest it unless you fully understand what earthing is all about, and how it improves safety under some circumstances, and lowers it under others).
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