ANSI-UL 60950:2000 pdf download Safety of Information Technology Equipment
Electric shock is due to current passing through the human body. The resulting physiologicaeffects depend on the value and duration of the current and the path it takes through the body.The value of the current depends on the applied voltage, the impedance of the source and theimpedance of the body. The body impedance depends in turn on the area of contact, moisturein the area of contact and the applied voltage and frequency. Currents of approximately half amilliampere can cause a reaction in persons in good health and may cause injury indirectly dueto involuntary reaction. Higher currents can have more direct effects, such as burn or ventricularfibrillation.
Steady state voltages up to 42,4 V peak, or 60 V d.c., are not generally regarded as hazardousunder dry conditions for an area of contact equivalent to a human hand. Bare parts which haveto be touched or handled should be at earth potential or properly insulated.
Some equipment will be connected to telephone and other external networks. SomeTELECOMMUNICATION NETWORKs operate with signals such as voice and ringing superimposed on asteady c VOLTAGE; the total may exceed the values given above for steady-state voltages. lt iscommon practice for the SERVICE PERSONNEL of telephone companies to handle parts of suchcircuits bare-handed. This has not caused serious injury, because of the use of cadencedringing and because there are limited areas of contact with bare conductors normally handledby SERVICE PERSONNEL. However, the area of contact of a part accessible to the usER. and thelikelihood of the part being touched, should be further limited (e.g. by the shape and location ofthe part).
it is normal to provide two levels of protection for usERs to prevent electric shock. Therefore, theoperation of equipment under normal conditions and after a single fault, including anyconsequential faults, should not create a shock hazard. However, provision of additionaprotective measures,such as protective earthing Or SUPPLEMENTARY INSULATON, is not considered asubstitute for, or a relief from, properly designed BASIC INSULATION.
Hazards may result from:
Contact with bare parts normally at HAZARDOUS VOLTAGES.
Breakdown of insulation between parts normally atHAZARDOUS VOLTAGEs and accessible conductive parts
Examples of measures to reduce hazards:
Prevent usER access to parts at HAZARDOUS VOLTAGES byfixed or locked covers,SAFETY INTERLOCKS,etc. Dischargeaccessible capacitors that are at HAZARDOUS VOLTAGES.
Provide BASIC INSULATon and connect the accessibleconductive parts and circuits to earth so that exposure tothe voltage which can develop is limited because
overcurrent protection will disconnect the parts havinglow impedance faults within a specified time; or provide ametal screen connected to protective earth between theparts，or provide DOLBLE Or REINFOACED INSULATION betweenthe parts, so that breakdown to the access ble part is notlikely to occur.