(Sub) CLAUSE/ANNEX HEADING:
OTHER RELEVANT (Sub) CLAUSES/ANNEXES (as applicable):
Clause 4.3 – Fault Conditions
DESCRIPTION OF ISSUE:
Fault conditions are applied in accordance with Sub-clause 4.3. The following will provide additional guidelines to assist in the application of the fault condition requirements. The guidelines themselves are not intended as requirements that must be met. Rather, they are ideas that may apply to a circuit in which failure of individual components may result in a compromise to the safety of the overall product.
The following techniques may be useful in minimizing the number of fault conditions:
Series Resistance – Eliminate components from test based upon review of impedances that will limit currents to non-critical values. For resistors located within a circuit, the following formula may be applied:
R = resistance value of the resistor
V = voltage across the resistor with the fault applied
P = wattage rating of the resistor
For example, testing may be waived in circuits supplied by a ¼ watt resistor located in a 35 volt supply provided that the resistor is rated at least 1.22 kilohms.
Bias conditions- Estimate the effect of an applied fault on the bias condition of involved semiconductor devices. For example, if a fault will turn off a circuit by shorting of the base-emitter junction of a transistor, perhaps the test can be waived.
Accumulated Test Data- Faulting of certain components within a circuit often yields information on the shut-down and operative characteristics of the entire circuit. Tests should be chosen in an order which may allow for subsequent reduced testing of other components.
Symmetry- Circuits should be examined for similarity, commonality or symmetry. Generally, only one leg of a symmetrical circuit need be tested.
Construction- Construction techniques should be reviewed to identify areas where tests can be minimized or eliminated. The dressing of leads, use of metal enclosures over circuits and proximity to non-flammable materials are examples of constructions that may help reduce the number of component fault tests.
Materials- Use of non-combustible (metal enclosed) or flame rated materials (such as UL94V rated plastics) may result in a reduction of tests.
Circuits- After a thorough analysis of the circuit and associated components, it may not be necessary to conduct abnormal fault testing in circuits supplied by a limited supply source (not more than 50 Watts into an external resistor). Examples of components in such a circuit are transistors, diodes and multiple pin integrated circuits.
The guidelines have been developed based upon common engineering analysis and experience in testing of similar circuits. Development was based upon typical circuits used in power supplies, but apply to any circuits used by the products covered by the scope of this standard.
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