Perhaps I should have mentioned for H.V work a cushy posh chair which conducts is the last thing one wants.........conflict with static yes.....to me a conductive arm band as is so often worn by assemblers to me in the type of R&D I do is the equivalent of a funeral in an electric chair.
The middle ground ? 1000000000ohm resistor ?
richj
The middle ground ? 1000000000ohm resistor ?
richj
One help when working with semiconductors might be to control room humidity. Very dry rooms will cause both more ESD and more corona issues. It may or may not also mean less dust, which has killed more than a few people. At high voltage you dont actually have to touch a conductor to be hit, it can reach out and grab you. It can reach a very long way if its dusty. I know of a man killed opening a dusty 460V panel.
Lots of caveats apply here, too wet is worse, and condensation is bad.
Lots of caveats apply here, too wet is worse, and condensation is bad.
The worst discharge I get is getting out of my Ford.....I'm absolutely licked when turn round to lock it....this far worse than any bolt I've had in my lab. A strap from underframe to ground doesn't last long. A non tribute to Ford in all these years not finding a better seating fabric.
richj
richj
Isolation Transformer & GFI
GFI
In electrical installations GFIs operate to disconnect a circuit whenever they detect that the flow of current not balanced between the phase conductor and the neutral conductor. The presumption is that an imbalance might represent a current leak through a person's body, a person who is accidentally touching the energized part of the circuit, who is grounded, and who is therefore about to receive a potentially lethal shock. These are designed to disconnect quickly enough to prevent these shocks.
Isolation transformer
An isolation transformer, often with symmetrical windings, which is used to decouple circuits. An isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass. In electronics testing, troubleshooting and servicing, an isolation transformer is a 1:1 power transformer which is used as a safety precaution. Since the neutral wire of an outlet is directly connected to ground, grounded objects near the device under test (desk, lamp, concrete floor, oscilloscope ground lead, etc.) may be at a hazardous potential difference with respect to that device. By using an isolation transformer, the bonding is eliminated, and the shock hazard is entirely contained within the device.
This material is from Wikipedia and looks to be pretty spot on as best I can tell.
GFI
In electrical installations GFIs operate to disconnect a circuit whenever they detect that the flow of current not balanced between the phase conductor and the neutral conductor. The presumption is that an imbalance might represent a current leak through a person's body, a person who is accidentally touching the energized part of the circuit, who is grounded, and who is therefore about to receive a potentially lethal shock. These are designed to disconnect quickly enough to prevent these shocks.
Isolation transformer
An isolation transformer, often with symmetrical windings, which is used to decouple circuits. An isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass. In electronics testing, troubleshooting and servicing, an isolation transformer is a 1:1 power transformer which is used as a safety precaution. Since the neutral wire of an outlet is directly connected to ground, grounded objects near the device under test (desk, lamp, concrete floor, oscilloscope ground lead, etc.) may be at a hazardous potential difference with respect to that device. By using an isolation transformer, the bonding is eliminated, and the shock hazard is entirely contained within the device.
This material is from Wikipedia and looks to be pretty spot on as best I can tell.
Good point about the jewellry. I was working on a car and my hand got stuck between the battery and the chassis. Took a while to free it, with my chromed watch bracelet merrily discharging the battery and getting hot and hotter. Took months for the burns to heal.
One bit of advice I was given long ago was only wear cotton if working with HV or gas. It doesn't stick to your skin in cases of extreme heat unlike manmade fabrics.
What size isolation transformer is best. Back in my TV days I had a massive one but always wondered if it was too big. All that current available to harm me. Is one just big enough to handle the load of the project safer?
One bit of advice I was given long ago was only wear cotton if working with HV or gas. It doesn't stick to your skin in cases of extreme heat unlike manmade fabrics.
What size isolation transformer is best. Back in my TV days I had a massive one but always wondered if it was too big. All that current available to harm me. Is one just big enough to handle the load of the project safer?
Re: Difference Between GRI Outlet and Isolation Transformer
A GFI (Ground Fault Interrupter) is designed to quickly disconnect the AC source within less than one half-cycle if "leakage" current is detected.
A little background...
In the US, domestic electrical service is supplied either as 220 volts or 120 volts. The 220 is really two 120 volt services, 180 degrees out of phase, but that's not important here.
What IS important is that for safety reasons (ironically enough!) one of the two wires is connected to earth ground. There really is a physical, actual, wired connection to a cold water pipe or a ground rod that has been hammered into the ground.
This is connected to the "neutral" wire (white) which goes to each outlet in your house.
The other wire in a 120v outlet is black, and is 120 vac "above" ground.
It's been this way for 100 years.
Now we all know that power transformers are expensive, right? Well the electronics industry knew that too, and they decided to make radios and televisions without power transformers.
This actually works, and is pretty safe -- AS LONG AS THE CHASSIS IS CONNECTED TO THE GROUNDED SIDE OF THE OUTLET! Way back when, it was a crapshoot, since the plug fit in the outlet either way, and the end user had a 50-50 chance of a safe family entertainment unit, or a family electrocution device.
After the first couple hundred wrongful-death lawsuits, the industry tried to make it easier to guess which way to plug the thing in. The first attempt was to place ribs along one side of the zipcord. The ribbed wire was connected to the chassis, and should be plugged into the grounded neutral of the outlet.
Since nobody knew this trade secret, a few thousand more electrocutions led to the familiar "polarized" plug, which has one blade wider than the other. The wide one is the grounded neutral. This had the added advantage that the matching outlet had a bigger slot for the grounded side, making it (slightly) more likely that Junior would stick the screwdriver/car keys/tongue into the grounded side of the outlet.
In order to make these devices safer, a n isolation transformer could be used. The transformer's secondary was not connected to earth ground in any way, and provided the same level of safety that would have been built in if the manufacturers had used a power transformer in the first place.
Blame it on the American tendency to buy the CHEAPEST unit possible.
The attached picture shows the typical isolation transformer. Meter M1 shows the "hazardous" voltage that should never be connected to the chassis (or the user!). Meter M2 shows the "safe" secondary connection. Meter M3 shows the other "safe" connection.
Note well, that isolation transformers are only useful for "transformerless" circuits, and since these are no longer in vogue (too many hungry lawyers), there is little need unless you are servicing a trenaformerless unit -- like most tube radios, phonographs and televisions made before 1970.
73 de Joe, W3BC
DrDeville said:Eye protection--great tip. Thanks!
I also appreciate the tip on using an isolation transformer on the workbench.
Could someone explain how an isolation transformer is different from the common Ground Fault Interupt outlets
George "Clueless" Ferguson
A GFI (Ground Fault Interrupter) is designed to quickly disconnect the AC source within less than one half-cycle if "leakage" current is detected.
A little background...
In the US, domestic electrical service is supplied either as 220 volts or 120 volts. The 220 is really two 120 volt services, 180 degrees out of phase, but that's not important here.
What IS important is that for safety reasons (ironically enough!) one of the two wires is connected to earth ground. There really is a physical, actual, wired connection to a cold water pipe or a ground rod that has been hammered into the ground.
This is connected to the "neutral" wire (white) which goes to each outlet in your house.
The other wire in a 120v outlet is black, and is 120 vac "above" ground.
It's been this way for 100 years.
Now we all know that power transformers are expensive, right? Well the electronics industry knew that too, and they decided to make radios and televisions without power transformers.
This actually works, and is pretty safe -- AS LONG AS THE CHASSIS IS CONNECTED TO THE GROUNDED SIDE OF THE OUTLET! Way back when, it was a crapshoot, since the plug fit in the outlet either way, and the end user had a 50-50 chance of a safe family entertainment unit, or a family electrocution device.
After the first couple hundred wrongful-death lawsuits, the industry tried to make it easier to guess which way to plug the thing in. The first attempt was to place ribs along one side of the zipcord. The ribbed wire was connected to the chassis, and should be plugged into the grounded neutral of the outlet.
Since nobody knew this trade secret, a few thousand more electrocutions led to the familiar "polarized" plug, which has one blade wider than the other. The wide one is the grounded neutral. This had the added advantage that the matching outlet had a bigger slot for the grounded side, making it (slightly) more likely that Junior would stick the screwdriver/car keys/tongue into the grounded side of the outlet.
In order to make these devices safer, a n isolation transformer could be used. The transformer's secondary was not connected to earth ground in any way, and provided the same level of safety that would have been built in if the manufacturers had used a power transformer in the first place.
Blame it on the American tendency to buy the CHEAPEST unit possible.
An externally hosted image should be here but it was not working when we last tested it.
The attached picture shows the typical isolation transformer. Meter M1 shows the "hazardous" voltage that should never be connected to the chassis (or the user!). Meter M2 shows the "safe" secondary connection. Meter M3 shows the other "safe" connection.
Note well, that isolation transformers are only useful for "transformerless" circuits, and since these are no longer in vogue (too many hungry lawyers), there is little need unless you are servicing a trenaformerless unit -- like most tube radios, phonographs and televisions made before 1970.
73 de Joe, W3BC
I think this is a given, but my power generation professor always used to empahsize this. If you're working with any sort of electricity, always always make sure your table is solid wood it is 1000% safer to use a solid wood table than any other table.
Also, if you feel something might get knocked over or you are testing and are scared to yank out wires, you can drill or screw in slots that wires can be looped around so when you tug on them they get locked or tightened into place.
I think the one-hand only rule is one of the most important lessons to learn.
Also, if you feel something might get knocked over or you are testing and are scared to yank out wires, you can drill or screw in slots that wires can be looped around so when you tug on them they get locked or tightened into place.
I think the one-hand only rule is one of the most important lessons to learn.
Any potential difference (voltage) that is more than 60volts across two points of your body is unsafe, be it on the secondary of an isolation transformer. Don't think putting one hand in the pocket plus insulating your shoes makes your other hand safe. This is due to the possibility of touching two points by this hand, with a higher unsafe voltage as in circuits with tubes.
Gajanan Phadte
Gajanan Phadte
Any potential difference (voltage) that is more than 60volts across two points of your body is unsafe, be it on the secondary of an isolation transformer. Don't think putting one hand in the pocket plus insulating your shoes makes your other hand safe. This is due to the possibility of touching two points by this hand, with a higher unsafe voltage as in circuits with tubes.
A bird sitting on an overhead line is safe until it does not touch the second line.
Gajanan Phadte
A bird sitting on an overhead line is safe until it does not touch the second line.
Gajanan Phadte