Hazardous Area and Explosion-Proof Load Cells: A Plain-English Guide

Some places can explode if a spark appears: grain silos full of dust, fuel depots, chemical plants, paint booths and mills. Equipment used there must be built and certified so it cannot ignite the surrounding gas or dust. A standard load cell is not designed for this. If you need to weigh a tank, hopper or silo in a hazardous area, you must use load cells and electronics that are approved for the hazardous zone, installed to the rules for that zone. This guide explains in plain language what a hazardous area is, what “intrinsically safe” and “explosion-proof” mean, how zones work, and what to ask your supplier. Always involve a qualified person, because the rules are strict and safety is at stake.

What is a hazardous area?

A hazardous area (also called an explosive atmosphere or classified area) is a place where flammable gas, vapour or dust can be present in amounts that could ignite. An explosion needs three things: fuel (the gas or dust), oxygen (the air), and a source of ignition (a spark or a hot surface). In a normal factory you control the third by keeping ignition sources out. In a hazardous area the first two are already there, so electrical equipment must not become the third.

Typical hazardous places include:

• Grain silos, flour mills and feed mills, where fine dust floats in the air.
• Fuel and chemical storage tanks.
• Paint and coating areas, where solvent vapour is present.
• Sugar, starch, wood and plastics handling.
• Pharmaceutical powder processing.
• Some mining and gas processing facilities.

Zones in plain English

To describe how often a hazard is present, standards divide hazardous areas into zones. Gas and dust have their own sets.

The lower the zone number, the more dangerous the area and the stricter the equipment requirements. A site safety professional or a hazardous area assessor decides which zones apply. You should never decide this yourself.

How equipment is made safe

There are several protection methods. The two that matter most for load cells are:

Intrinsic safety (often marked “Ex i”)

The idea is to limit the electrical energy in the circuit so that even a fault cannot create a spark strong enough to ignite the atmosphere. The load cell, which has very low power, is a natural fit. A safety barrier or isolator sits in the safe area and limits the energy sent down the cable into the hazardous area. The whole circuit, cell, cable and barrier, is designed and checked as a system.

Flameproof or explosion-proof enclosures (often marked “Ex d”)

The equipment is housed in a strong enclosure that can contain an internal explosion and stop flames escaping. This is more common for motors, junction boxes and instruments than for load cells themselves.

Other methods exist, such as dust ignition protection by enclosure for dust areas. The approach must match the zone, the gas or dust group and the temperature class of the site.

Schemes and markings: IECEx, ATEX and FM

You will see several approval schemes on datasheets.

• IECEx and AS/NZS 60079 series: the international scheme and the matching Australian and New Zealand standards, commonly used for hazardous area work in Australia.
• ATEX: the European scheme, widely recognised and often used alongside IECEx.
• FM Approvals (FM): a United States scheme that classifies hazardous locations in classes and divisions, for example Class I, II and III Division 1.

Zones and divisions are not the same system, so approvals do not map one-to-one. Always check which approvals your site and your regulator accept, and confirm that the equipment markings match the zone you need. A hazardous area specialist can advise.

Some load cells in our range are offered in hazardous-area versions. For example, the Sensortronics 60063 S-type notes versions approved for hazardous areas under FM Class I, II, III Division 1, and the Sensortronics 65016-W shear beam is listed with FM approval. Whether a particular version suits your Australian zone depends on its approvals and on the system as a whole, so talk to us before ordering.

A simple example: a grain silo

A grain handler wants to weigh a feed silo. Grain dust is combustible, so the area around the silo is classified as a dust zone by a qualified assessor.

1. Zone defined. The assessor sets the dust zone and the temperature limits.
2. Cells chosen. The supplier offers load cells with hazardous area approvals that suit the zone, with sealed stainless steel construction. See tank, silo and hopper weighing for sizing.
3. Barrier fitted. A certified safety barrier or isolator sits in the safe area, and the indicator is also outside the hazardous zone.
4. Cables run correctly. The cable is the right type and length, within the limits on the system certificate, and is routed and sealed as required.
5. Installed and inspected. A qualified person installs, inspects and signs off the work.
6. Maintained. The site keeps records and re-inspects regularly.

What to tell your supplier

To help us recommend the right equipment, have this information ready.

• The substance: gas, vapour or dust, and its name or group.
• The zone and the temperature class set by your assessor.
• The approval scheme your site requires (IECEx, ATEX or other).
• The capacity, number of cells and mounting arrangement.
• The environment: outdoor, washdown, corrosive, extreme heat.
• The electronics you plan to use, and where they will be mounted.

Common mistakes

• Using a standard load cell because it is “low voltage”. Low voltage does not make a device safe. It must be certified.
• Mixing certified and uncertified parts. The system certificate covers specific combinations.
• Ignoring cable limits. Intrinsic safety depends on cable capacitance and inductance, which depend on length.
• Forgetting the junction box. It must be approved too.
• Letting maintenance change the system. Swapping a part without checking the certificate can void safety.
• Treating dust as harmless. Fine organic dust can explode violently.

Good housekeeping in dust areas

Safe equipment is only part of the answer. Dust layers on load cells and mounting hardware can act as insulation and hold heat. Keep cells and the surrounding structure clean, remove dust build-up regularly, and follow your site’s hazardous area management plan.

Questions to ask before you start

1. Is the area really classified, and has a qualified person documented the zones and gas or dust groups?
2. Could the weighing be done outside the hazardous area, for example by weighing a delivery vehicle on a weighbridge instead of the vessel itself?
3. Do the cells, junction box, barrier and cable all have certificates that cover the combination you plan to use?
4. Who will inspect and maintain the system, and are they trained for hazardous areas?

Sometimes the simplest answer is to move the measurement into a safe area. When that is not possible, the certified route is the only acceptable one.

Keep the paperwork

Hazardous area equipment comes with certificates, drawings and instructions that form part of the safety case. Keep them with the system. Record the cell serial numbers, the cable lengths, the barrier model and the date of each inspection. If a part is replaced, record that too. During an audit or an incident investigation, good records are evidence that the system was maintained as designed.

Earthing, static and the structure

Static electricity can ignite some atmospheres, so vessels and supports in hazardous areas are usually bonded and earthed according to the site rules. Mounting hardware for load cells can interrupt the electrical path to earth, so the design must allow for bonding straps where needed. Raise this with your electrical engineer early, because it can affect the choice of mounting.

Frequently asked questions

Can any stainless steel load cell be used in a hazardous area?

No. Stainless steel is about corrosion resistance. Hazardous-area use requires specific certification.

Is an IP68 load cell explosion-proof?

No. IP ratings describe dust and water resistance, not explosion protection. See the load cell glossary.

Who decides the hazardous zone?

A qualified person, such as a hazardous area assessor, in line with the standards and your site’s risk assessment.

Can I install the equipment myself?

Hazardous area installation generally requires qualified, competent people. Check the rules for your state and site.

How do I start?

Tell us about your site using the contact page. We can discuss suitable load cells, and you can use the complete load cell guide to prepare.

Important note

This article is general information, not safety or engineering advice. Hazardous area work must follow the applicable standards, regulations and your site’s requirements, and be carried out by qualified people.