I got the feeling the OP was referring to always putting the device under test on an isolation transformer, rather than the scope.
No one has the guts to say just pull out the ground plug. You could get a 3 foot outlet strip and take pliers and rip out the ground prong then plug the transformer into the strip. NO GROUND. Why open the transformer and monkey around with wires?
But this is like saying puppies are bad.
Just to clarify a few things.
It is never a good idea to float a scope by using an isolation transformer or cheater plug. This can induce a lot of additional stress on the equipment. Also, if an electrical occurs failure within the scope, the chassis may be elevated to line potential. This introduces a serious shock hazard.
I see nothing wrong with using an isolation transformer to float the circuit under test. There is however, a right and a wrong why do do this. If you simply connect a standard isolation transformer to your mains powered equipment (DUT) while keeping the ground connection intact, you aren't really achieving any isolation at all. Using an isolation transformer in this way only serves to filter out EMI and other undesirable artifacts present on the mains. This is why they are used so extensively in hospitals to protect sensitive medical equipment from interference.
If you truly want to "isolate" the mains powered circuit you are analyzing, you need to remove the ground connection on the output of the isolation transformer while keeping the ground connection on your scope intact. At this point, if you measure from either end of the output winding to ground, you will see 0 VAC. The only way to receive a shock using this method is to simultaneously contact both output wires of the transformer. This is was you might call an isolation transformer for "technical purposes". The danger of smoking your scope by connecting the earth lead to an energized part of the equipment has been eliminated. The risk of shock has also been greatly reduced.
If you don't believe me take a few measurements with the ground connection intact and then lifted on your isolation transformer. With the ground intact, you will measure full line voltage from either end of the secondary winding to ground. Now lift the ground on the transformer. Measure from each end of the secondary winding to ground. That mains voltage that you just measured is no longer present. Probing either end of the winding to ground will not show any voltage because the winding is no longer referenced to earth ground. The only way to measure mains voltage in this scenario is to probe directly across the secondary winding. Presto, instant isolation.
If you truly want to "isolate" the mains powered circuit you are analyzing, you need to remove the ground connection on the output of the isolation transformer while keeping the ground connection on your scope intact. At this point, if you measure from either end of the output winding to ground, you will see 0 VAC. The only way to receive a shock using this method is to simultaneously contact both output wires of the transformer.
Just to clarify a few things.
It is never a good idea to float a scope by using an isolation transformer or cheater plug. This can induce a lot of additional stress on the equipment. Also, if an electrical occurs failure within the scope, the chassis may be elevated to line potential. This introduces a serious shock hazard.
I see nothing wrong with using an isolation transformer to float the circuit under test. There is however, a right and a wrong why do do this. If you simply connect a standard isolation transformer to your mains powered equipment (DUT) while keeping the ground connection intact, you aren't really achieving any isolation at all. Using an isolation transformer in this way only serves to filter out EMI and other undesirable artifacts present on the mains. This is why they are used so extensively in hospitals to protect sensitive medical equipment from interference.
If you truly want to "isolate" the mains powered circuit you are analyzing, you need to remove the ground connection on the output of the isolation transformer while keeping the ground connection on your scope intact. At this point, if you measure from either end of the output winding to ground, you will see 0 VAC. The only way to receive a shock using this method is to simultaneously contact both output wires of the transformer. This is was you might call an isolation transformer for "technical purposes". The danger of smoking your scope by connecting the earth lead to an energized part of the equipment has been eliminated. The risk of shock has also been greatly reduced.
If you don't believe me take a few measurements with the ground connection intact and then lifted on your isolation transformer. With the ground intact, you will measure full line voltage from either end of the secondary winding to ground. Now lift the ground on the transformer. Measure from each end of the secondary winding to ground. That mains voltage that you just measured is no longer present. Probing either end of the winding to ground will not show any voltage because the winding is no longer referenced to earth ground. The only way to measure mains voltage in this scenario is to probe directly across the secondary winding. Presto, instant isolation.
No one has the guts to say just pull out the ground plug. You could get a 3 foot outlet strip and take pliers and rip out the ground prong then plug the transformer into the strip. NO GROUND. Why open the transformer and monkey around with wires?
But this is like saying puppies are bad.
here most things don't have ground and in older house there are no ground in the outlets
The problem is that anything with a switch mode supply the gnd/chassis will usually float at 110V via the input filter capacitors
so you have to be very careful if anything is grounded because it will zap most inputs[/quoteb
Where is here? You do not have your country flag posted.
I also live in an old house with no grounds and some members cannot believe this
Their response is I have to re-wire the house.
Quotehere most things don't have ground and in older house there are no ground in the outlets
The problem is that anything with a switch mode supply the gnd/chassis will usually float at 110V via the input filter capacitors
so you have to be very careful if anything is grounded because it will zap most inputs[/quoteb
Where is here? You do not have your country flag posted.
I also live in an old house with no grounds and some members cannot believe this
Their response is I have to re-wire the house.No grounds here ether. The place is nearly 65 years old.
The place is nearly 65 years old.
Quotehere most things don't have ground and in older house there are no ground in the outlets
The problem is that anything with a switch mode supply the gnd/chassis will usually float at 110V via the input filter capacitors
so you have to be very careful if anything is grounded because it will zap most inputs[/quoteb
Where is here? You do not have your country flag posted.
I also live in an old house with no grounds and some members cannot believe this
Their response is I have to re-wire the house.
Denmark, I think it wasn't until after mid 1975 that outlets had to have ground, but since almost everything comes with a schuko plug that doesn't connect ground
in a danish outlet almost nothing is grounded
QuoteThe place is nearly 65 years old.
I am 67 and my house is 102 years old. We both are showing our age. The front of the house still has the original wiring and the back half was re-wired in the 70's (still no grounds). The original wiring is thick (maybe 10 ga) covered with heavy cloth. It is very hard to work with. In the 90s I connected some ceiling fans and really hate to touch this wiring. So I will do anything to avoid touching an outlet (like running extension cords all over the place)
If I get an isolation transformer, it will also be an old one (no ground) so there is no question about a ground.
Incorrect.
I don't know what you're measuring mate, but an isolation transformer it is not!
Incorrect.
I don't know what you're measuring mate, but an isolation transformer it is not!
It always amuses me when someone has just enough time to call foul, but never enough time to explain their reasoning. Thanks for the laugh MATE!
I've used a fully isolated variac (NOT an autotransformer) over the past 10 years mainly as an isolation transformer when testing mains powered devices.
I always wondered if I would get zapped from its secondary if I only touched one of the wires, but never dared to test it for real, until today.
First I used a low input impedance DMM (3 kOhm) to measure the voltages between isolated and unisolated mains, and as you might expect, these were very low (much higher in the normal high Z position, of course). Then I tried a neon screwdriver on the secondary side. Hmmm, it lit, does it need so little current that the capacitive leakage between primary and secondary is enough for it to light?