Millivolt Drop Test Procedure

What is the Millivolt Drop Test?

A millivolt drop test is a procedure used to determine the electrical integrity of electrical connections & contacts, which are commonly found in 

• Circuit breakers (or) 
• Switchgear. 

It requires measuring the voltage drop in millivolts across a connection at a known current to find high resistance concerns or poor contact quality.

A millivolt drop tester detects the voltage drop across 2 test locations. 

In simple terms, it is a D.C digital voltage generator that will determine the voltage drop across the test points at a fixed current. 

It usually has 2 scales for amperes: 0 to 20 amps & 10 to 100 amps. 

The voltage ranges are: 19.9, 199.9, & 1999 mV.

Purpose

To determine the electrical reliability of a circuit breaker’s connections and contacts. 

This can be accomplished by running a millivolt drop test across each pole’s line and load terminals with circuit breaker contacts closed. 

The millivolt drop of a circuit breaker pole can vary greatly due to inherent variability in the extremely low resistance of the 

• Electrical contacts & 
• Connectors. 

Such variances do not always indicate inadequate performance & must not be utilized as the main factor for determining acceptability. 

The millivolt drops test can be very complicated, and these changes do not always indicate poor performance. 

The Molded Case Circuit Breaker (MCCB) standard, UL 489, 

Ex: It does not need specific millivolt drop values. Rather, the standard addresses the temperature of the terminals & other external breaker locations under continuous current conditions. 

Thus, the millivolt drop test is meant only to assess the thermal performance of the circuit breaker.

Equipment Required

• 24 V  DC Supply
• Circuit Breaker (MCCB)
• Millivolt Drop Tester

Procedure

• Set the breaker to OFF mode. Turn off power to breaker to isolate it from other circuits.
• Open the enclosure. To ensure that the equipment is de-energized, check for no voltage on incoming conductors (& control power wires if present) and between them and ground. 
• To complete these requirements, use the procedures outlined in NFPA 70E Part II.
• Remove the breaker from enclosure.
• If the circuit breaker has an under-voltage trip release, activate it for proper operation.
• If the circuit breaker has an under-voltage trip release, activate it for proper operation.
• Apply test current to a pole equal to circuit breaker rating (or 500 A minimum for breakers rated over 500 Amperes). 
• Keep a record of the millivolt drop & the test current. Do not stay current for longer than one minute.
• The IEEE publication “Measuring Molded Case Circuit Breaker Resistance” demonstrates that millivolt drops can decrease over time with continuous current. 
• The UL temperature rise test requires long-term current flow and equilibrium conditions.
• More than a minute. Thus, the preceding test will give an indication of millivolt drop, but will not measure the equilibrium value.
• De-energize the test circuit. Manually turn the breaker OFF, then ON.
• Repeat steps obtain three readings on the pole being tested.
• Repeat steps for each of the circuit breaker’s remaining poles.

Result

The data will vary depending on breaker frame type, ampere rating, and manufacturer. 

Consult the manufacturer to determine an appropriate level for the breaker under test. 

If average test values on any breaker pole exceed the manufacturer’s specifications, it may indicate an overheating condition that requires more testing. 

Circuit breaker and measurement variables, including 

• Source voltage, 
• Current, 
• Stability, and 
• Duration, 

can impact test findings. 

These causes millivolt drops to be higher than those measured under continuous alternating current duty. 

The exterior temperatures determined under equilibrium conditions should be used to determine the breaker’s continuous use. 

If the average millivolt drop exceeds 200 under the following test settings, it is recommended to communicate with the original equipment manufacturer (OEM).

Advantages

• The test identifies locations of excessive resistance in the circuit that might cause voltage drops, overheating, & component failure. 
• Assessing contact quality ensures proper connections and low resistance. 
• Early detection of excessive resistance can avert equipment failure and safety issues. 
• The test is quick and easy to execute, allowing for early detection of faults.

How to calculate Millivolt Drop?

The formula can be used to calculate the appropriate voltage drops for cables that operate under direct current (DC) conditions. 

For 3 Phase: 2 x Route Length x Current x Resistance x 10^-3 (in Volts)

Why is the Millivolt Drop method used for measuring Low Resistance?

A conductor that has a low resistance is ideal for high currents; but, if the current is not managed, the I²R will be higher, and the conductor will become damaged.