There are always rules. And where safety is concerned, it is absolutely critical to abide by them.
The core safety standard that applies to ICT (Information and Communications Technology) equipment operating from the AC line or battery supplies of up to 60 V is IEC/EN/UL60950-1. The standard encompasses telecommunications infrastructure equipment regardless of its power source, in addition to components and sub-assemblies such as power supplies that have intrinsic safety implications. It describes appropriate measures to assure safety, such as the need for isolation barriers and protective grounding.
The standard's guiding principle is to provide two levels of protection from electric shock and energy hazards that may trigger other dangers, such as fire. This two-level model creates a hierarchy of protection measures that build upon five categories of insulation: functional; basic; supplementary; double; and reinforced. Functional insulation is necessary for the product to function properly, and it may reduce the likelihood of ignition and fire hazards, but it provides no reliable safety protection from electric shock.
For ICT equipment, the output side of isolated board-mounted power supplies (BMPS) normally meets the criteria for a Safety Extra-Low Voltage (SELV) circuit that limits voltages to a safe level of 60Vdc maximum under both normal operation and single-fault conditions. The isolation requirements that the power brick must then satisfy depend on the level of isolation that the AC/DC front-end power supply provides, together with the system's arrangements for connection to protective ground.
Generally, the AC/DC power supply in ICT equipment has reinforced or double insulation (alternatively basic and/or supplementary insulation) between the AC line supply and its DC output. Furthermore the output and the input of the BMPS connect to protective ground, which means that the system already includes two levels of protection from electric shock and energy hazards. If the normal input voltage exceeds 60Vdc, it must additionally withstand an electrical strength test and pass fault-condition testing.
The isolation voltage in the electrical strength test depends on the input voltage and the working voltage and typically ranges from 1000 to 1400Vdc. In fact, the great majority of commercially available isolated DC/DC modules specify 1500Vdc isolation, which has become the de-facto industry standard.
But does Power-over-Ethernet (PoE) change the scenario? PoE, of course, distributes network power via Ethernet data cables, and according to the IEEE 802.af3 standard requires a high degree of electrical isolation -- in fact, 2250Vdc is necessary to meet the requirements. Typical equipment applications receiving power via Ethernet cable are usually remote connected devices such as VoIP telephones, WLAN access points and network cameras. Given the significantly increasing deployment of PoE in communication installations with power supply lines that traverse outside environments, such as links between buildings, could this mean the isolation voltage for telecom power generally increasing to 2250Vdc?
Ericsson is certainly preparing with the recently launched PKU5510E 50W DC/DC converter module that targets PoE applications and features an ultra-wide input range of 18V to 75V and has built-in functional isolation of 2250Vdc. But is the safety net about to be made a little bigger?