Monday: Intrinsic Safety (IS) Barrier Bypassing
Why bypassing a blown IS barrier diode on a live process loop instantly defeats hazardous area protection.
When a 4-20mA instrument loop drops out in a Class 1 Div 1 hazardous area, pressure builds to get operations back online fast. If troubleshooting reveals a blown Zener diode in the Intrinsic Safety (IS) barrier, the temptation is to temporarily bypass the barrier with a jumper wire.
Never bypass an Intrinsic Safety barrier.
Intrinsic Safety is a design philosophy that prevents explosions not by containing them inside heavy cast-iron enclosures (like explosion-proof ratings do), but by ensuring there is fundamentally not enough electrical energy available to ignite an explosive atmosphere—even under fault conditions.
The Zener barrier inside the marshalling cabinet acts as an absolute energy throttle. If you bypass it, the loop continues to function normally, hiding the fact that you have just turned an intrinsically safe instrument into an active ignition source. If that field instrument shorts or sparks, the full energy of the 24VDC power supply will now rush into the hazardous area without restriction.
If a barrier blows, replace it with an identical, rated unit. A jumper wire is just a fuse waiting for an explosive atmosphere.