Post on 10-Nov-2014
195TECHNOLOGY FOR SAFETY
Appendix 5 – INTRINSIC SAFETY MINIMUM IGNITION CURVES
MINIMUMIGNITION
CURVES
The graphs on the following pages answer the
question: What is a dangerous amount of elec-
trical energy?
These graphs are for circuits containing alumi-
num, cadmium, magnesium or zinc–substances
that produce a high temperature incendiary
spark.
It is important to keep in mind that these cur-
ves reflect the worst case scenario.
When designing intrinsically safe electronic
equipment today, most manufacturers start by
specifying the equipment for the worst possible
case.
The graphs chosen are those that are used
most often by designers and manufacturers of
electrical apparatus.
196 TECHNOLOGY FOR SAFETY
Appendix 5 – INTRINSIC SAFETY MINIMUM IGNITION CURVES
Minimum Ignition Curves for Inductive Circuits
Inductive CircuitsMinimum igniting currents appli-cable to electrical apparatus withcadmium, zinc, magnesium or alu-minium where U = 24 V.
The maximum permissible work-ing current will be two thirds ofthe values shown.
* Groups A, B, C and D are Class INorth American classifications.
** Currently there is no NorthAmerican equivalent to theEuropean Group I classification.
METHANEGroup I **
PROPANEGroup IIA (D)*
ETHYLENEGroup IIB (C)*
HYDROGENGroup IIC (A&B)*
197TECHNOLOGY FOR SAFETY
Appendix 5 – INTRINSIC SAFETY MINIMUM IGNITION CURVES
Minimum Ignition Curves for Resistive Circuits
Resistive CircuitsMinimum igniting currents appli-cable to electrical apparatus withcadmium, zinc, magnesium or alu-minium.
The maximum permissible work-ing current will be two thirds ofthe values shown.
METHANEGroup I **
PROPANEGroup IIA (D)*
ETHYLENEGroup IIB (C)*
HYDROGENGroup IIC (A&B)*
198 TECHNOLOGY FOR SAFETY
Appendix 5 – INTRINSIC SAFETY MINIMUM IGNITION CURVES
Minimum Ignition Curves for Capacitive CircuitsGroup I
C + 40Ω (Cd)
C + 15Ω (Cd)
C + 5.6Ω (Cd)
C + 0Ω (Cd)
Capacitive CircuitsMinimum igniting voltages (22 percent hydrogen) applicable toGroup I electrical apparatus.
The curves correspond to valuesof current limiting resistance asindicated.The curve marked Sn is applicableonly where cadmium, zinc or mag-nesium can be excluded.
The maximum permissible work-ing voltage will be two thirds ofthe values shown.
C + 0Ω (Sn)
199TECHNOLOGY FOR SAFETY
Appendix 5 – INTRINSIC SAFETY MINIMUM IGNITION CURVES
Minimum Ignition Curves for Capacitive CircuitsClass I, Groups A & B (Group II, C)
C + 40Ω (Cd)
Capacitive CircuitsMinimum igniting voltages appli-cable to Class I, Groups A and B(Group II, C) electrical apparatus.
The curves correspond to valuesof current limiting resistance asindicated.The curve marked Sn is applicableonly where cadmium, zinc or mag-nesium can be excluded.
The maximum permissible work-ing voltage will be two thirds ofthe values shown.
C + 15Ω (Cd)
C + 5.6Ω (Cd)
C + 0Ω (Cd)
C + 0Ω (Sn)
200 TECHNOLOGY FOR SAFETY
Appendix 5 – INTRINSIC SAFETY MINIMUM IGNITION CURVES
Inductance - mH
Min
imum
Ign
itin
g C
urre
nt -
Am
ps
H y d r o g e n
Certification Curves Showing RelationshipBetween Inductance and Minimum Igniting Current