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# Method for Identifying a PoE Power Supply Failure Caused by High Network Cable Resistance Publication Date:  2015-06-15  |   Views:  1067  |   Downloads:  0  |   Document ID:  EKB1000078990

Contents

## Issue Description

Applicable Models and Versions

The method applies to all AP models and software versions. ## Handling Process

Method 1:

Measurement tool:

Multimeter

Measurement contents and procedure:

Use the multimeter to measure the DC resistance of each cable wire and take down the measured values as R1, R2,…R8.

Measurement results:

Network cable resistance:

R = (R1 + R2 + R3 + R6)/4 (when the switch uses the wires 1, 2, 3, and 6 to supply power.)

Or:

R = (R4 + R5 + R7 + R8)/4 (when the switch uses the wires 4, 5, 7, and 8 to supply power.)

Usually, the AC and PoE switch use wires 1, 2, 3, and 6 for power supply, and the PoE adapter uses wires 4, 5, 7, and 8 for power supply.

Method 2:

Measurement tool:

Multimeter

Measurement contents and procedure:

Connect one end of the network cable to the AP.

Use the multimeter to measure the cable resistance on the other end.

R1: resistance between wire 1 and wire 2

R2: resistance between wire 3 and wire 6

R3: resistance between wire 4 and wire 5

R4: resistance between wire 7 and wire 8

Measurement results:

Network cable resistance:

R = (R1 + R2)/4 (The switch uses the wires 1, 2, 3, and 6 to supply power.)

Or:

R = (R3 + R4)/4 (The switch uses the wires 4, 5, 7, and 8 to supply power.)

Usually, the AC and PoE switch use wires 1, 2, 3, and 6 for power supply, and the PoE adapter uses wires 4, 5, 7, and 8 for power supply.

Method 3:

Measurement tool:

Multimeter

Measurement contents

Measure the input voltage on the AP.

Measure the voltage of the AP-side network cable (V1) and output power of the switch (P1).

Measure the output voltage on the switch.

Continuously measure the output power of the switch. When the measured output power is approximately the value of P1, measure the voltage of the switch-side network cable (V2).

Measurement results:

Take down three measured values:

V1, V2, and P1

Calculation method:

Step 1: Calculate the current of the network cable I (I ≈ P1/V2).

Step 2: Calculate the voltage of the network cable V (V = V2 - V1).

Step 3: Calculate resistance of the network cable R (R = V/I).

## Suggestions

According to 802.3af PoE standards, the DC resistance of the network cable cannot exceed 20 ohms (The power consumption of the powered device cannot exceed 12.95 W).

According to 802.3at PoE standards, the DC resistance of the network cable cannot exceed 12.5 ohms (The power consumption of the PD cannot exceed 30 W).

For any questions, contact Huawei technical support engineers.