Electronics > Beginners
Isolation transformer ground and safety questions
<< < (3/5) > >>
rs20:

--- Quote from: Shock on May 16, 2015, 11:02:07 pm ---For instance, cutting of a ground/earth pin for a device plugged straight into the wall would isolate the ground/earth between the wall and the device only. But it would eliminate the protection of a residual current circuit breaker device installed in the room or switchboard.

--- End quote ---

I'm being super-pedantic here, but the RCD still protects (or at least activates) against touching live in the device with one hand and touching any other earthed device/water pipe, right? I'm just nitpicking one point, everything else you said is spot on.
madires:
In most power distributions N (neutral) is earthed at the transformer station or pole. If your house got a two wire wiring, i.e. L and N, N has the potential of PE (earth) plus the voltage drop caused by the current through the N wire. In a 3 wire setup, i.e. L, N and PE, it's the same but PE provides a low impedance path to earth, since it doesn't pass any significant current, just a few mA from line filters, like in large SMPSUs, and leaky devices. Because N is earthed, L is also referenced to earth.

An isolation transformer for an electronics lab provides floating mains. Actually you don't have any mains N or L anymore, just a simple AC output. It's like a 12V transformer. If the metal enclosure of the isolation transformer is connected to PE or not doesn't matter for the floating secondary. With a 2 wire mains you don't have PE anyway. With a 3 wire mains it becomes interesting. In case of a failure in the primary side, the earthed enclosure would protect you from an electrical shock and the RCD would be triggered (PE is an ideal path to earth) or the breaker. But it would also increase the risk, that a problem (loose wire) with the floating secondary could "earth" the secondary. The same goes for the secondary's socket, when it got a contact for PE too. A problem with the DUT (device unter test) or a bad wiring could earth the floating secondary. And you don't want this to happen. The isolation transformer I got is double isolated and has no PE, neither at the primary nor at the secondary, though it got a metal enclosure.

If you have to use your scope please get differential probes or use the poor man's way (ch1 + ch2 inverted, and ground clips removed).

BTW, another good video:


Mark Hennessy:

--- Quote from: madires on May 17, 2015, 11:51:40 am ---A problem with the DUT (device unter test) or a bad wiring could earth the floating secondary. And you don't want this to happen.
--- End quote ---

Of course, as soon as you connect test gear to the DUT, then it is no longer floating. But that is OK because the DUT is earthed-referenced at the point you wish to make your measurements with respect to.

But this is why only one DUT can be connected to an isolation transformer.

It's all too easy to assume the isolation transformer gives you more protection than it actually does (http://en.wikipedia.org/wiki/Risk_compensation).
queennikki1972:

--- Quote from: madires on May 17, 2015, 11:51:40 am ---In most power distributions N (neutral) is earthed at the transformer station or pole. If your house got a two wire wiring, i.e. L and N, N has the potential of PE (earth) plus the voltage drop caused by the current through the N wire. In a 3 wire setup, i.e. L, N and PE, it's the same but PE provides a low impedance path to earth, since it doesn't pass any significant current, just a few mA from line filters, like in large SMPSUs, and leaky devices. Because N is earthed, L is also referenced to earth.

An isolation transformer for an electronics lab provides floating mains. Actually you don't have any mains N or L anymore, just a simple AC output. It's like a 12V transformer. If the metal enclosure of the isolation transformer is connected to PE or not doesn't matter for the floating secondary. With a 2 wire mains you don't have PE anyway. With a 3 wire mains it becomes interesting. In case of a failure in the primary side, the earthed enclosure would protect you from an electrical shock and the RCD would be triggered (PE is an ideal path to earth) or the breaker. But it would also increase the risk, that a problem (loose wire) with the floating secondary could "earth" the secondary. The same goes for the secondary's socket, when it got a contact for PE too. A problem with the DUT (device unter test) or a bad wiring could earth the floating secondary. And you don't want this to happen. The isolation transformer I got is double isolated and has no PE, neither at the primary nor at the secondary, though it got a metal enclosure.

If you have to use your scope please get differential probes or use the poor man's way (ch1 + ch2 inverted, and ground clips removed).

BTW, another good video:


--- End quote ---



So if I understand this correctly...

I have a house built in 1947 with no PE (Earth Ground).

I am purchasing a Isolation Transformer that has no chassis ground on the secondary side and when I use it on a two wire plug, (which has GFCI), I can have one DUT on the transformer?

As for the Oscilloscope, I know you should not float your scope, however I am also plugged into GFCI with no PE, which does not protect my equipment and I should use either differential probes or use the poor man's way (ch1 + ch2 inverted, and ground clips removed)?

Did I get that right?
madires:

--- Quote from: queennikki1972 on August 21, 2019, 01:05:39 am ---I have a house built in 1947 with no PE (Earth Ground).

I am purchasing a Isolation Transformer that has no chassis ground on the secondary side and when I use it on a two wire plug, (which has GFCI), I can have one DUT on the transformer?

--- End quote ---

Yes, the DUT would be floating as long as you don't create a connection between the DUT and PE or anything conducting to earth or the soil around/below your house.


--- Quote from: queennikki1972 on August 21, 2019, 01:05:39 am ---As for the Oscilloscope, I know you should not float your scope, however I am also plugged into GFCI with no PE, which does not protect my equipment and I should use either differential probes or use the poor man's way (ch1 + ch2 inverted, and ground clips removed)?

Did I get that right?

--- End quote ---

Yep, it's the recommended way to reduce the risk of an electric shock.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod