According to US-headquartered Perle Systems, the problem arises particularly when PoE extenders are used to stretch PoE networks beyond the 100m limit, typically using existing, but redundant, copper wires – such as former alarm or telephone circuits. These extenders can add up to 3km to the reach of a network, allowing it to power remote items such as sensors, routers and WiFi access points, especially if they are in locations where there is no other convenient source of power.
“PoE Ethernet extenders enable you to reach these devices, up to 10,000 feet away, and power them over single twisted-pair, coax or any other existing copper wiring,” explains Al Davies, Perle’s director of product management.
The drawback is that most PoE Ethernet extenders operate as simple passive power injectors, applying 44–57V to the RJ-45 port pins. This allows up to 30W of power to be transferred efficiently to PoE-compatible equipment via the cable, while remaining at a safe level for users. But it can result in severe damage to non-PoE-compliant Ethernet devices if they are connected accidently, possibly costing thousands of dollars.
In the past, users demanded that manufacturers of PoE equipment such as switches and media converters put safety checks in place to eliminate such damage. The manufacturers have achieved this by using power management controllers that meet the low-voltage requirements of the Safety Extra Low Voltage (SELV) part of the IEEE 802.3af/at standard.
“Built-in PoE management controllers are the difference between a powered device under control and a disaster waiting to happen,” says Davies.
Until recently, however, there haven’t been any PoE extenders that support the SELV standard and would thus protect non-PoE equipment attached to the extension. Perle says that it is now offering the only extenders that comply with IEEE 802.3af/at and will thus allow both PoE and non-PoE devices to be attached safely.
By injecting a low voltage into the cable – using a process known as signature detection – the extenders can determine whether or not attached Ethernet devices are PoE-capable. If not, then the higher voltage is not applied. The extenders support both end-span and mid-span power sources.
Perle has produced a technical note explaining the issue in greater depth.