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For security professionals: How to correctly identify whether a PoE switch provides standard PoE power supply

Release time： 2025-12-22 14:30

To understand how to determine whether a PoE switch is providing standard PoE power, one must first clarify what a PoE switch is.

A PoE-powered switch refers to a switch that can provide network power to remote power-receiving terminals through network cables. It includes both a network switch and PoE power supply functions, and is a common power supply device in the PoE power supply system.

Classification of POE Switches2

1、Standard POE switch

Compliant with IEEE 802.3af and 802.3at standards, it is a standard POE switch (commonly known as 48V). The complete POE consists of two parts: the power supply end device (PSE, Power Sourcing Equipment) and the power receiving end device (PD, Power Device). The POE switch is a type of PSE device. PSE is the device that supplies power to the terminal and is also the manager of the POE Ethernet power supply process. PD is the terminal device that receives power (48V national standard devices contain PD chips).

2、There are two types of non-standard POE switches.

Instead, it refers to non-standard POE switches ("non-standard" is abbreviated as "non-standard", commonly known as 24V, but non-standard also includes 48V, 24V, 12V, 5V, etc.).

The main application fields of POE switches

The POE switch mainly provides network and power supply for devices such as monitoring (for cameras), wireless coverage (for APs), and wireless transmission (for bridges).

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The working process of a standard POE-powered switch

1、Detection: The POE switch initially outputs a very small voltage. Once it detects the power receiving device (referred to as "PD" for "Power Delivery" device that supports the IEEE 802.3at/af standard chip) with the chip that complies with the IEEE 802.3at/af standard.

2、PD classification: After detecting a PD, the POE switch classifies the PD device and assesses its power.

3、Power Supply: During the startup period (generally less than 15 microseconds), the POE switch supplies power from a low voltage to the PD, until it reaches the voltage required by the PD, providing stable and reliable direct current to the PD devices.

4、Power outage: If the PD device is disconnected, the POE switch will quickly (generally within 300 to 400 milliseconds) stop supplying power to the PD and will repeat the detection process to check if the terminal of the cable is connected to the PD device.

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Testing methods for identifying genuine and counterfeit POE using a multimeter

Start the equipment, set the multimeter to the voltage measurement position, and use the two probes of the multimeter to touch the power supply terminals of the PSE device (usually terminals 1/2, 3/6 or 4/5, 7/8 of the RJ45 port). If a stable output voltage of 48V or other values (such as 12V, 24V, etc.) is measured, the device is a non-standard product. Because during this process, the PSE does not test the power receiving device (which is the multimeter in this case), but directly supplies 48V or other voltage values.

Conversely, if the voltage cannot be measured and the pointer of the multimeter jumps between 2V and 10V, it indicates a standard PoE. Because at this stage, the PSE is conducting a test on the PD terminal (in this case, the multimeter), and since the multimeter is not a legitimate PD, the PSE will not supply power and no stable voltage will be generated.

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