Checking through 110v DC control supply

[clarkgrizwald]

Is there an approved method of checking through a 110v DC control supply using a multi-meter?
I normally check with one lead of the meter connected to earth, and check for voltage at various points throughout the circuit, but an engineer at work says this is not the correct method.
Does anyone have any views on this?
clarkgrizwald

 

[itsmoked]

There are two wires that may be a the same earth potential.

The safety ground and the current carrying neutral. If you are checking the circuit/devices operation you should be checking from the neutral to the various points in the circuit. If you are checking safety aspects of the ground lead then use it(earth).

 

[ScottyUK]

Typical 110V DC industrial supplies for protection or essential services are ungrounded. This allows a single ground fault to occur somewhere on the system without causing any disruption the operation of the system. A monitor to detect this condition is normally installed so that a ground fault can be traced and remedied before a second fault occurs. Occurence of a second ground fault elsewhere usually results in consequences ranging somewhere from a blown fuse to a fire.

If you measure an ungrounded system relative to earth, the measurement is essentially meaningless because there is no other reference to earth - you should measure relative to one or other pole of the supply. Most people use the negative pole. If you do get any readings measuring to earth then you may have a ground fault. If your system is 'clean' the voltage you may measure will slowly fall to zero as the input resistance of the meter drains the stored charge from the system capacitance. So in essence, if your system is ungrounded then the other guy is right - sorry! - and if the system is intentionally grounded then your technique should work.



----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[GTstartup]

In actual fact ungrounded 110VDC circuits with a ground detection scheme will have a voltage to ground to each pole. Normally 55VDC. Measuring to ground from + pole will be +55 and from negative -55V. These measurements are a good way to tell if you have a ground fault even if its not direct.

Bottom line is, you should do both pole to pole and pole to ground.

 

[ScottyUK]

Hmm,

Kinda depends on the fault detector - some inject low frequency AC onto the ungrounded supply, and that gives some wierd results. The Alstom MR625 (and MR627 I think) use this technique. My multimeter was not impressed!

If your system is clean, the 55V or whatever should gradually decay as the system capacitance drains through the meter impedance. If the voltage slowly decays all the way to zero, the chances are you have no ground faults.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[clarkgrizwald]

Thanks for the replies everyone, I will need to find out if our 110v DC system is grounded or not.
The circuit I was checking at the time was valve actuator limit switches, and the neutral conductor was not within reach for testing. That was why I was using the earth connection.
clarkgrizwald