Appendix: Loop and cable condition test
March 2014 – Revision 2.0
Page 70 of 74
3M™ Drive-Thru Systems Model G5 and Model XT-1
Installation Manual
Appendix: Loop and cable condition test
Measure loop inductance/resistance for each order point, recommend
replacement in comments section if needed. You must conduct two
separate tests on the Inductance Loop to determine it’s condition:
1. Lineal Conductor Resistance and Inductance
This test measures the Resistance and the Inductance of the loop conductor or
wire at the Base Station.
Procedure:
To measure the inductance and resistance of the loop wire, use test
equipment such as an LCR meter (Inductance [L], Capacitance [C], and
Resistance [R]).
A reading of 1.2 Ω for resistance and 100–120μH (or 20μH / wire turn) for
inductance are acceptable measurements that reflect good wire condition.
2. Insulation Resistance
In addition to the factors mentioned above, ice, water, frost, etc. can contribute
to the change of the conductor’s resistance. However, measuring the conductor’s
resistance alone may not provide a complete picture, as it does not account for
insulation leakage to the ground/earth. Insulation leakage can be checked by
utilizing a Megohm meter (or a Megometer or Megger Meg Out, in industry
parlance) capable of testing up to 500 megohms (500 million ohms).
In this test, a DC voltage is applied to the loop wire to check for insulation
damage that may cause a short circuit when normal power is applied. The
Meggers have settings capable of delivering anywhere from 500–1000 volts
through the loop. DO NOT apply more than 500 volts for this test or you
may damage the insulation. The cable’s insulation is rated at 600 volts.
Procedure:
Step 1: Disconnect the loop wires from the loop detector so the loop is floating.
Step 2: Place one of the meter leads to the loop wire and the other to the
ground/earth..
Resistance above 20 megohms: Acceptable
Resistance below 20 megohms: Loop wire insulation has been damaged.
Loop integrity is questionable
Resistance less than 10 megohms: Loop wire must be replace
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