Interpreting the Volt/Ohm Meter

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 by: Don McCarty
(This article originally ran in the February 2013 issue of OSP Magazine)

Mr. McCarty,
First, I read all of your columns because it has improved my technical knowledge and improved my troubleshooting skills in the outside plant from the C.O. to the N.I.D. With IPTV testing, is 20 meg ohms sufficient as a minimum resistance measurement? I have been mentored/instructed to accept 150 meg ohms T-R, R-G and T-G. Let me know what your thoughts are on this subject.
—Terence

Terence,
When we qualify a cable pair for The Triple Play we are looking for a clean cable pair with good longitudinal balance within the reach of The Triple Play service. We expect the cable pair to be free of DC type faults such as crossed battery, a short or ring or tip ground, with no series resistance such as a popped scotch lock connector or a bad pica bond connector. We expect a capacitive balance between tip and ring to ground to be greater than 98%.

That being said, crossed battery (when testing a vacant tip or ring a VOM indicates battery from another working ring) and resistance can be a grey area. When we first started testing in the telephone business we used analog meters that were not too accurate.

Crossed Battery

In the old days when testing a vacant pair for crossed battery, the DC voltmeter used a 500 K ohm/volt network. If testing across a pair tip to ring this was acceptable. The voltmeter was in parallel with the applied voltage and a correct reading was obtained. A friend and expert in the business told me long ago to test for voltage in parallel and test for current in series. When we test for crossed battery tip or ring to ground we violate that rule, placing meter resistance is in series with the fault, and the indicated battery on the volt/ohm meter shows less than applied.

When testing the vacant pair tip or ring to ground the meter was in series with the unwanted battery source and unless the path to the unwanted voltage was 0 ohms the indicated voltage would be less than the applied voltage.

Example: A vacant tip conductor is crossed up with -51VDC from another working ring and the unwanted path was 0 ohms the Voltmeter would show -51VDC. If the resistance of the unwanted path from the other circuit was 270k-ohms the meter would indicate -18VDC, less than applied.

If the meter presented 500k-ohm to the circuit the voltage across the 270k would be 18 volts and that is what you are reading. The meter being in series with the 270k-ohm plus the added 500k-ohm meter resistor forming a 770k-ohm series circuit, the meter resistance drops the other 33 volts. Without the meter in the circuit, and no current, that vacant tip would be floating at -51 VDC.

Today’s Volt/ohmmeters use a 1 Meg ohm resistor in the ohms/volt network, but the indicated voltage would still read less than -51VDC in the above example, maybe 8 volts. Larger resistance in meters provides less loading to the circuit when measuring in parallel. In series it adds to the resistance, and provides a larger voltage drop across the meter itself.

There is a wrinkle to this that makes it not so simple. Digital meters now may use field effect transistors in some form controlling an amplifier circuit as the input to the meter, rather than a current limiting resistor. But, in any case, using a voltmeter in series has to make the meter a voltage drop of some size or other.

Remember: Telco’s pay field technicians great deals of money to find and fix unwanted paths from other circuits and when tested after repair ring and tip to ground should show 0 volts. Nothing other than 0 volts is acceptable.

Resistance

When testing for a short or ring or tip ground, the internal battery in the ohms selection was either a 15-volt or a 45-volt battery supply. This was not enough voltage to ionize high resistance faults, but for Plain Old Telephone Circuits (POTS) it was acceptable and few repeat reports occurred. Most of our cables were pulp-and-paper insulated so a cable pair tested clean (greater than 30 Meg ohms) or faulted in the k ohm range. When Plastic Insulated Conductors (PIC) were introduced many of our water-associated faults would ionize when ringing voltage (105VAC) was placed on the circuit. Repeat reports went through the roof.

To circumvent this problem a higher voltage (150VDC) was applied when testing for resistive faults. High-resistance faults in the Meg ohm range were ionized and then showed in the k ohm range. These faults could then be proactively located with a quality resistance bridge. Newer multifunctional test sets raised the threshold for identifying high resistance troubles. The upper limit of the ohmmeter was moved to first 100 Meg ohms and then to 999 Meg ohms. This higher range allowed field technicians to identify and proactively repair faults before customer service was affected. The DSL business raised the bar for resistive faults from 3.5 Meg ohms to 20 Meg ohms, which is the upper limit for the newest resistive fault locators.

The problem with IPTV is when there is a ring or tip ground in the 20 Meg ohm range ringing voltage will ionize the fault making the path 0 ohms and knock down the set top box. The customer states: “When my phone rings my set-top box drops.”

I set the limit to 20 Meg ohms for this reason: If the fault is more solid than 20 Meg ohms it can be located with a resistive fault locator. If the fault is higher than 20 Meg ohms the field technician must use the “divide and conquer” method of fault locating. Cut the pair in half and see which way the trouble is.

Signing off
I appreciate comments, questions and challenges sent in by you loyal readers. Keep them coming! Contact me at dmccarty@mccartyinc.com or 831.818.3930.

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