measurement equipment guard

[wieserfalke]

Hello,
I recently came over the topic of measurement equipment guards an didn’t found very much Information about it.

What I already think that I know:
    • It is there to prevent leakage currents for devices connected to mains.
    • To implement an guard the transformer needs a shield between the primary and secondary windings

What I hope someone can explain to me:
    1) What this guard is about?
    2) When I need an guarded measurement?
    3) How to measure something guarded?
    4) How to implement an Guard in an DIY measurement equipment?
    5) In winch type of measurement equipment do I need to implement an guard?

Thanks for your Answer.
Greetings
Tobias Falk

[TimFox]

Take a guarded ammeter, for example:  the guard terminal is forced (by the meter) to the same potential as the measuring contacts.  Therefore, if you don't want the current from another part of the circuit to contribute to the measured current, you connect that node to the guard.  It is not the same as "ground".
As another example, the DE-5000 LCR meter (among others) connects the device under test to two or four (non-grounded) terminals.  Again, the guard terminal allows you to remove the impedance to another node from the measurement.  For example, if you have a delta connection of three capacitors, connect the guard to one corner of the triangle and you will only read the capacitance between the other two, ignoring the series combination of the other two capacitors in the delta.

[jeffjmr]

Ack!

You had me following until the delta capacitor example.

Seems to me you could only test the one cap opposite the guard. Or is one of the test leads co-located with the guard? In which case which two caps are under test?

Apologies if this is a hijack.

Jeff

[coppercone2]

the guard is forced to x voltage so you put node A at x volts, I assume this is a AC guard, which matches phase?

not sure how a guard works for a AC signal like a LCR meter, with phase and all, I just know for DC

you basically connect a gain of 1 amplifier to the signal pin to drive the guard with a buffered version of what its guarding. the voltage is forced at low impedance. if there is 3 capacitors, one node (the one that is being measured) will cause the guarded node to mimic that voltage.

[TimFox]

Ack!

You had me following until the delta capacitor example.

Seems to me you could only test the one cap opposite the guard. Or is one of the test leads co-located with the guard? In which case which two caps are under test?

Apologies if this is a hijack.

Jeff

In the Delta case with three capacitors:  If you connect the guard to terminal 3, then you measure the capacitance from 1 to 2.  If you move the guard to terminal 2, you measure from 1 to 3. etc.