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