Why my PoE powered device cannot work properly when connected to the PoE Switch？

PoE is short for the Power over Ethernet, which has many technical advantages, such as cost saving, simplicity, mobility, controllability and etc. By using this technology, we can provide direct current to the devices that based on IP network without changing anything of your original network’s structure cabling.

• PSE: Power Source Equipment

It refers to the devices that can supply DC power to the PDs through Ethernet cable. PSEs can automatically search, detect, classify the PDs and supply power to the PDs.

• PD: Powered Device

The PDs are divided to standard devices and nonstandard devices. The standard PD refers to the devices compatible with the IEEE802.3af/at. The standard PDs can be powered by the standard PSE while nonstandard PDs are used with matched PSE.

• 1) IEEE802.3af standard

IEEE802.3af standard defines the maximum output power from a single port to be 15.4W, and it can supply power to the devices under 12.95W (such as IP camera) through Ethernet cable.

• 2) IEEE802.3at standard

IEEE802.3at standard is based on IEEE802.3af and can be regarded as the enhanced version of the IEEE802.3af. The power from a single port is up to 30W, which can meet the requirement of the higher power devices such as video Phone, PTZ device and etc.

• 3) The difference between the two standards.

There are two power supply mode based on IEEE802.3af/at standard.

• Alternative A: Supply power using the date line pairs 1/2 and 3/6 of the Ethernet cable
• Alternative B: Supply power using the spare line pairs 4/5 and 7/8 of the Ethernet cable

1) Alternative A:

In the picture above, you can see that the PSE supplies power using date line pairs 1/2 and 3/6, and polarity of the line pairs can be positive or negative, which is decided by the designation of chip vendors.

2) Alternative B

In the picture above, the line pairs 4/5 and 7/8 are free lines and the 4/5 pair is positive pole while the 7/8 pairs is the negative pole.

Note:

1. According to IEEE 802.3af/at protocol, the standard PSEs can just support either.Alternative A or B, while the standard PDs must support both modes and be compatible with all kinds of the polarities.
2. At present, all of TP-Link’s PoE switches are standard PSE and designed based on the Alternative A and the line pair 1/2 is passive pole while the line pair 3/6 is the positive pole.

Q1: Why my PD fails to work when connected to the PoE switch and the LED light on the switch isn’t on?

A: Troubleshooting steps:

1. Make sure the PD is a standard device based on IEEE802.3af/at protocol.
2. Ensure that you have chosen the right and qualified Cat.5 or above Ethernet cable and plug into the right port on the PD. And the length of the Ethernet cable cannot be over 100m.
3. For some switches, not all of the RJ-45 ports are PoE ports, you can refer to the UG for more information.
4. Confirm the power supply mode with the PD’s vendor, there may be two situation that cause the problem:

• 1) The PD only support Alternative B. But as mentioned above, TP-Link’s PoE switches are based on Alternative A. So the PD and PSE cannot cooperate together.
• 2) The PD support Alternative A, and the line pair 1/2 is passive pole and the line pair 3/6 is positive pole, as introduced in last chapter, the PD cannot work either based on using the straight through cable normally. Howerver, you can use a crossover cable to avoid the problem.

So if the PoE LED does not lit up on the switch after you plug you PD into the PoE port and your PD can’t be powered on, it is recommended to contact your PD’s vendor and ask for the PD’s power supply mode and the polarities it supports after eliminating the basic network factors. This information is also necessary when you contact us for support.

Q2: when my PD works with the PoE switch, the indictor LED light flickers at times (gets on, but dies quickly and for a while it goes to the loop), why?

A: The LED light gets on, it means that the PD can be powered, but for some reasons, the PSE stops the process.

It is likely that the total output power of the PSE exceeds or approaches the maximum output power. You can check the status of the PoE MAX light, if it is on or blinking, it means the power is insufficient. You can also try the stand-alone test to debug the problem.

It could be due to that the link-loss detection mechanism using by the switch and your PD device are not compatible with each other. At present, all of TP-Link PoE switches only support DC link-loss detection mode. If your PD only supports AC link-loss detection mode, it's very likely not working with the switch. So when the phenomenon exists but the PoE MAX light is normal, you can contact your PD’s vendor to know the link-loss detection mode the PD supports. This information is also important for us to troubleshooting.

Q3: Why my PD can work with other vendor’s PSEs but cannot work with TP-Link’s PoE switches?

A: As stated above, we should first make sure the PD is a standard device and confirm the power supply mode, polarities and link-loss detection mode it supports. Some devices can only work with the products produced by the same manufacturer; some nonstandard devices may be able to cooperate with nonstandard devices if the parameters are matched. These products may have a problem when cooperating with other standard PSEs, so it is highly recommended to know the protocols that the devices is based on before you decided to purchase one.

Note:

Although some vendors also claim their PDs are compatible with IEEE802.3af/at, they have implied some nonstandard (or their own) technology and these PDs can only cooperate with their own PSEs. These PDs may also have problems when work with other vendors’ standard PSEs.

The status of the LED lights on the PoE switches is very important when judging the problems and troubleshooting. There are small differences about the meanings of the light status in different series of the switches, you can refer to the UG for more specific information.