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Hi guys I have a few questions regarding my findings today and wondering if any of you could help one of the guys asked me to have a look at an electrical panel due to him finding voltage from a 24v PSU + to the earth of the panel and the - to earth each readings were 6vac which I was curious why we’d have a reading maybe a floating voltage? The others question that made me think was there was a loose earth from one of the Siemens level control units that was sparking. I checked the panel earth which was all good and live and neutral. I then disconnect the earth to the seimens unit and measured voltage between the seimens earthing terminal to the panel earth which was 110v ac what would this be? I couldn’t work out if there was a fault on the unit which was making the earthing terminal 110v but no fuses were blowing when contacted which also made me think Weather the 110v is being induced via the electronics?
 
Regarding the Siemens unit, most electronic devices have some sort of interference supression fitted across their mains input, a simple example being the capacitor arrangement below. If you disconnect the earth, put crudely, you end up with a capacitor divider across the 240v mains, so you see 110v on the now disconnected earth, given your meter is so high impedance it doesn't load the circuit much.

Industrial control panel 1632488791895 - EletriciansForums.net
Don't really understand the first part of the question, sorry?
 
Most stuff leaks a small current to earth in normal operation, or has the ability to do so if you complete the circuit to earth with your multimeter from an otherwise floating supply. A normal meter with 10 megohm input impedance absorbs only 0.0006 milliamps when reading 6V, which is a very tiny current compared to what can be capacitively coupled across a power supply unit. So if your 24V circuit is supposed to be floating and you are seeing 6V AC to earth with such a meter, it is likely to be a classic example of a 'ghost voltage' i.e. one from a source of very high impedance that can only deliver a trivial current. In such situations the actual voltage displayed by the meter doesn't tell you very much, as it depends as much on the input resistance of the meter as it does on the source of the 'ghost.'

A more informative test is to measure or calculate the current itself. Obviously if there is a fault and you try to measure the current with an ammeter, you run the risk of shorting the supply. You can use one of the testers with a deliberately low input resistance range that will load down the source, or shunt the input to your meter with a suitable resistor and see how that changes the reading. From that you can calculate the leakage source impedance, and hence the current that would flow if one side of the floating supply were grounded.

All of this is probably over-stating the situation. I would be surprised not to find a floating supply riding on at least some AC to earth.


Re. the 110V, as Avo clearly explains, it's common for the disconnected ground lead of a delta capacitor suppressor to hover around half the mains volts, usually slightly lower, due to the symmetry of the capacitors forming a 2:1 divider.
 
Most stuff leaks a small current to earth in normal operation, or has the ability to do so if you complete the circuit to earth with your multimeter from an otherwise floating supply. A normal meter with 10 megohm input impedance absorbs only 0.0006 milliamps when reading 6V, which is a very tiny current compared to what can be capacitively coupled across a power supply unit. So if your 24V circuit is supposed to be floating and you are seeing 6V AC to earth with such a meter, it is likely to be a classic example of a 'ghost voltage' i.e. one from a source of very high impedance that can only deliver a trivial current. In such situations the actual voltage displayed by the meter doesn't tell you very much, as it depends as much on the input resistance of the meter as it does on the source of the 'ghost.'

A more informative test is to measure or calculate the current itself. Obviously if there is a fault and you try to measure the current with an ammeter, you run the risk of shorting the supply. You can use one of the testers with a deliberately low input resistance range that will load down the source, or shunt the input to your meter with a suitable resistor and see how that changes the reading. From that you can calculate the leakage source impedance, and hence the current that would flow if one side of the floating supply were grounded.

All of this is probably over-stating the situation. I would be surprised not to find a floating supply riding on at least some AC to earth.


Re. the 110V, as Avo clearly explains, it's common for the disconnected ground lead of a delta capacitor suppressor to hover around half the mains volts, usually slightly lower, due to the symmetry of the capacitors forming a 2:1 divider.
Thank you AVO and Lucien this explains why it wasn’t blowing a fuse and My earth loop tester came up with error interesting stuff thanks again guys!!
 
I’m still trying to get my head round when we connect the earth to these bits of equipment are we not then raising the local earth potential due to being 110Volts?
 
I’m still trying to get my head round when we connect the earth to these bits of equipment are we not then raising the local earth potential due to being 110Volts?
You need to consider the impedance of the components here. The capacitors I mentioned that create the 110v are very very high impedance (think resistance) and the ground very very low impedance (resistance). So considering the ratio of these impedances is many thousands to one, the potential of the ground is raised by perhaps a millivolt, or maybe a microvolt or two. So you are right about the concept, but think about the details of the 'circuit'.?
 
You need to consider the impedance of the components here. The capacitors I mentioned that create the 110v are very very high impedance (think resistance) and the ground very very low impedance (resistance). So considering the ratio of these impedances is many thousands to one, the potential of the ground is raised by perhaps a millivolt, or maybe a microvolt or two. So you are right about the concept, but think about the details of the 'circuit'.?
Oh I get it feel silly asking that now ?? but yeah similar concept to having a high r1 value and a low R2 value I guess. Potential dividers
 

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