Earth lead in mains plug supplying sciope.

Earth lead in mains plug supplying scope.

If you are a beginner please do not interfere with the earth connection in your instruments in any way. It is very dangerous.

Two major electronics companies that i worked at cut the earth from mains plugs supplying oscilloscopes., -though i am told it is highly dangerous.
-these are amongst some of the biggest electronics companies around.


I currently have a mains powered Tektronix oscilloscope from which i have cut the earth in the mains cable.

I did this so that i can take differential voltage measurements without the ground clip having to be at earth potential....
-basically i don't want to destroy expensive prototypes that i spend ages making.

....Whenever i touch the earth clip, i get a slight shock.

What is the electrical path of my elctrocution?

Is it capacitively via the Y cap that bridges the isolation barrier between primary and secondary inside the scope.?

Differential Scope Primer Introduction : Tektronix

https://www.tek.com/Measurement/App_Notes/Technical_Briefs/tds3000-float/eng/51W_10640_1.pdf

**broken link removed**

As explained, if there is a mains earth connection to the scope it implies the ground clip on the probe is also earthed. While that is not an issue when probing isolated equipment, if you wanted to look at mains connected circuits, such as the primary side of a switch mode PSU, the very last thing you should do is earth the live components. Oscilloscopes are normally fitted with transformers chosen for low capacitance and high insulation resistance between windings to minimize coupling to the mains but inevitably there is still some leakage and it can eventually charge enough to give a mild shock. At the risk of letting some current go the wrong path, a trick I use is to wire a resistance of about 1M Ohms between the earth wire and earth pin. I make it from two series resistors of 470K - 560K or thereabouts as this gives less capacitance than using one and it lessens the chance of flash-over, bearing in mind that most resistors are only rated to work up to 250V. It's enough to discharge the leakage to a safe level without risking damage to the circuits.

Brian.

The suggestion are most likely helpful. Strictly spoken, an oscilloscope isn't designed as a class II instrument with safe insulation but as class I instrument requiering a protective earth. So the only safe way compliant with safety regulations and acceptable to occupational insurance associations to operate it without PE is to use an external safety transformer. The difference matters in case of an accident, when someone may be hold responsible for it.

I agree with FvM that an oscilloscope with a designed-in earth connection as its 0v reference should not have its earth connection removed, exactly for the reasons stated in the above post.

However, there are 2 solutions - both using an external isolation transformer on the mains supply:
It would be possible to operate the oscilloscope from an isolated supply before working on circuits where differential readings are required, but it might be an even safer solution to leave the oscilloscope connected as intended by the manufacturer, and to supply the equipment-under-test from an isolated mains supply.

If the equipment being tested is connected to the mains through an isolation transformer, then you can create a connection to earth at any reference point you wish. It also means that any contact you make with any single part of the circuit will not result in an electric shock.
I recommend fitting a good isolation transformer to the workbench of everyone who works on high voltage equipment !

thankyou,

One place i used to work had mains isolation transformers (shaped like heavy yellow blocks with a handle on)

-the strange thing was, the secondary plug connection on the isol. txformer, actually carried through the mains earth connection, so we had to physically cut the earth connection in the plug that we connected to the isol txformer secondary.

i am wondering why the earth conenction would be in the secondary's plug connection?...surely an isol txformer is for isolating away from earth?

Also, betwixt i appreaciate your informative answer, though i am not sure what exactly is "charging" and how....i presume its the prim-sec Y cap?.....or some stray capacitance between prim and sec?

eem2am said:

. . . mains isolation transformers (shaped like heavy yellow blocks with a handle on)

-the strange thing was, the secondary plug connection on the isol. txformer, actually carried through the mains earth connection, so we had to physically cut the earth connection in the plug that we connected to the isol txformer secondary.

i am wondering why the earth conenction would be in the secondary's plug connection?...surely an isol txformer is for isolating away from earth?

Click to expand...

Very interesting, and a good question.
Its interesting from the point of view of anyone who would be performing a Risk-Assessment of the work.
By disconnecting the earth AFTER an isolation transformer supplying the equipment under test, then the protection offered by any RCD devices on the supply to the workplace is lost. Anyone making a personal current path between two points with a high-voltage difference would be at a high risk of injury.
But conversely, any equipment under test which is isolated from the mains supply AND has had its earth connection removed, will present little risk to someone making contact with earth (the local supply earth or a structural earth) and ANY part of the equipment. (Another hazard does arise, though, if any other grounded test equipment is connected to the unit (which is other wise floating).

The risks always have to be understood and evaluated again for each situation.