A load cell installed in a zone with flammable dust or gas needs more than a certificate. Intrinsically safe systems are defined by the combination of cell, barrier and cable, and a certified cell on an uncertified barrier is not a safe installation.
The system concept
An intrinsically safe circuit limits the energy available in the hazardous area, using a barrier or galvanic isolator in the safe area. The cell must be rated for the zone and gas group, and the barrier must have entity parameters compatible with the cell: voltage, current, power and capacitance. The cable adds capacitance and inductance, which count against the allowance.
Common mistakes
Using a standard junction box in the hazardous area, running the cable through a zone without accounting for its capacitance, and mixing cells from different certifications on one barrier. Each of these invalidates the assessment even though every individual item carries a label.
Documentation
Keep the system certificate, the barrier datasheet and the cable parameters together, and record the calculation. Inspectors ask for the loop calculation, not the cell certificate. For dust zones, also check surface temperature and the ingress protection of the enclosure.
Kevin Huang