UL-60950-21:2016 pdf free download – STANDARD FOR SAFETY Information Technology Equipment – Safety – Part 21: Remote Power Feeding.
A.3 Safety considerations
It is not intended for the USER to have access to an RFT CIRCUIT. The USER should be protected against electric shock as specified in IEC 60950-1 by limiting access to an RFT CIRCUIT as if it were a SECONDARY CIRCUIT at HAZARDOUS VOLTAGE; however, higher spacings and insulation requirements may be necessary, as an RFT CIRCUIT is considered to be subject to overvoltages.
SERVICE PERSONS are protected against contact with RFT CIRCUITS, and large area contact with bare parts of RFT-V CIRCUITS, by insulating these circuits as SECONDARY CIRCUITS at HAZARDOUS VOLTAGE.
If SERVICE PERSONS touch both conductors of an RFT-C CIRCUIT, most of the current available to provide power to the RFT-C CIRCUIT load equipment or equipments (IRFT) could pass through the body. Therefore, to protect under these conditions, ‘RET should not exceed the limits that cause no harmful physiological effect, taking into account the hand-to-hand current path through the body (60 mA).
To protect SERVICE PERSONS who touch one conductor of an RFT-C CIRCUIT and are earthed through the feet, the maximum permitted body current is less, due to passing close to the heart. The current from one conductor of the RFT-C CIRCUIT to earth under normal operating conditions can easily be limited and should not exceed the current that usually causes no reaction (2 mA). If one conductor of the RFT-C CIRCUIT is accidentally earthed and the person touches the other conductor, the ‘RR should not exceed the limits that cause no harmful physiological effect (25 mA), taking into account the hand-to-foot current path through the body. If ‘RFT exceeds this value, there should be a mechanism, triggered by the unbalance created by the fault, to reduce it to 25 mA or lower.
SERVICE PERSONS should also be protected against the electric shock that may be caused by discharging the charged capacity of the RFT-C CIRCUIT through the body (see A.5.2) to comply both with the operational requirements and also with the safety requirements.
Because the human body is in general more tolerant of d.c. current than of a.c. current, there is more design flexibility to comply with the operational requirements and also the safety requirements if d.c. is used. Therefore this standard does not specify requirements for a.c. remote power feeding.
Power to the RFT-C CIRCUIT supply equipment is supplied by the AC MAINS SUPPLY or by a d.c. source.
The remote power feeding current is limited by current control in the RFT-C CIRCUIT supply equipment to the maximum value ‘RFT = 60 mA. In the event of a fault between one conductor of the circuit and earth, the current must comply with a lower limit, if necessary by the use of a monitoring and control device.
The remote power feeding voltage URFT depends on the length and resistance of the TELECOMMUNICATION NETWORK and the power requirements of the RFT CIRCUIT load equipment. No voltage limit is specified for protection against electric shock because this protection is given by the current limitation (current source concept).
However, URFT cannot exceed 1 500 V because surge arrestors on the TELECOMMUNICATION NETWORK (see lEG 60950-1, Clause 6, Note 1) limit the voltage to this value. In practice, the operating voltage of surge arrestors and the voltage rating of the cable may enforce the use of a lower value.