I would appreciate the group opinion on how to implement Interlock
circuitry on a PCB (printed circuit board)
The equipment have to comply with UL1950/CSA950,EN60950. The Interlock
protects against severe mechanical and laser radiation hazards. The
interlock system comprises of circuitry implemented on PCB.
THE ISSUE IS: What spacings should be kept between printed conductors that
carry the Interlock signals, and other printed conductors on the PCB ? The
Interlock signals are SELV and are 5 volts or 24 volts, but, if short
circuit occurs between printed conductors it will fail the Interlock
system !!
I think that REINFORCED insulation can be adopted for this case, as only
this kind of insulation is relied upon not to become shorted. Therefore,
CLEARANCE of 0.8 millimeters (from Subclause 2.9, Table 5; condition 4,
Pollution degree 2) is required by the standard.
The required CREEPAGE (Table 6) should than be 2.4 millimeters for 50 volt,
and (by interpolation between 0 volts and 50 volts) 1.2 millimeters for 24
volt, 0.3 millimeters for 5 volt.
FIRST QUESTION: Is this interpolation OK ?
SECOND QUESTION: Components (Integrated circuits) that comprise the
interlock system have pin spacings of less than 1.2 millimeters. This
makes keeping 1.2 millimeters impossible. We could consider to use
conformal coating on the PCB, than a spacing of just 0.2 millimeters is
required (subclause 2.9.5, table 7) but this much complicates the
production process !!
Is there another way ?
Please Comment !
