Hi all

I was wondering when measuring for a DC ground fault in a live circuit is it safe to say you measure each lead of the dc circuit (+\-) to ground if there is no voltage? there is no fault but any voltage to ground would indicate the fault??

Cheers thanks for the help.

 

Anyone?

 

Hi,

Ground fault is when one of the live wires touches ground resulting in current flowing to ground. That means you will get voltage difference between live wire and ground. Ground loop will provide path to lower impedance, which will draw higher current.

Fault will occur when the ground current exceeds maximum loop current.
As to measuring, i'm not sure, generally protectors(GFP, or RCD or GFCI) are used.

 

This explanation presumes the DC supply is ungrounded, with a possibility of a ground reference circuit (basically a voltage divider network consisting of two resistors of equal value in series, the center of which is grounded). The reason you may not have received a response is we don't know enough about the system you are working on.

When there is a ground on one leg and you are measuring that leg with respect to ground, then there is zero potential between the leg with the ground and ground.

The other, ungrounded, leg when read with respect to ground will then read full voltage--because it does not have a ground.

Again, this refers to a DC supply system as described above.

 

Thanks for the reply. i should have detailed more in my first post so. basically on a 24vdc digital output to a relay or transmitter, when measuring from the positive to ground you should show 0 vdc (as well on the negative lead in respect) on your DMM on a non grounded circuit. but on a grounded circuit you will show a potential difference, correct? The only reason I am confused is that particularly on VAC when measuring from hot to ground you can see the full 120Vac

 

John,

This is more common than you may expect. AC voltage can show up on DC wires if they run closely to each other or in the same raceway. It can be caused by EMI (emitted Magnetic Interference) caused by the transformer effect of current in an AC wire inducing a voltage in adjacent wires or RFI (radio Frequency Interference) caused by the capacitive effect of adjacent wires with a difference of voltage coupling noise.

The solution is the same no matter where the AC noise is coming from, Simply connect a 1,500 ohm 1 watt resistor between your +24 volt wire and ground and if that does not clear it up completely from your DC system, connect one between the -24 volt wire and ground as well.

Search pull-down resistor on the web and you will find this solution for various voltages and applications.

This works well for most any noise on inputs to PLCs.

I hope this helps,
William Hinton