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Explosion-proof and intrinsically safe

A device that is explosion-proof is obviously not as good as stopping an explosion in the first place, but not everything can be made intrinsically safe. As Barry Atkins of PASS explains, with regards to test instrument enclosures, there are some significant differences

The design of an explosion-proof tester, device or unit is such that any explosion that is caused by a flammable gas entering the device is contained within it and does not leave the casing, to prevent an internal spark from causing a much larger explosion.

The case itself is tested to ensure that it would not heat up sufficiently to ignite any surrounding gas. Explosion-proof devices are usually large, heavy and made from stainless steel or cast aluminium.

Intrinsically safe means that any electronic device or wiring will not have the power or potential to cause any type of spark or arcing. Intrinsically safe devices such as process calibrators cannot produce a surface temperature high enough to cause ignition.

Cameras

It is advisable to get advice when it comes to cameras as it can be difficult to choose between intrinsically safe (which could be overkill) and explosion-proof (which could suffice). In Europe, CENELEC and IEC standards refer to protection methods with symbols, such as EX d for explosion-proofing.

There are pros and cons for each protection concept that will dictate the route a manufacturer would take to design, certify and manufacture an instrument. If a device requires a significant amount of power to operate or its construction cannot be controlled to a component level, then explosion-proofing may need to be used. For any non-metallic enclosure materials, anti-static materials must be used, regardless of whether the protection concept is intrinsically safe or explosion-proof.

When an explosion-proof device is tested for compliance with ATEX for Europe, the flame path gap is twice that required for US certification. Specifically, the maximum gap in the US for Gas Group B is 0.05mm whereas the maximum gap for the equivalent Gas Group in Europe is 0.1mm.

A key difference between Europe and North American certification requirements is the mix of gases used to create the sample explosion during testing. This different gas mix results in a different set of pressure parameters, in particular where pressure piling is concerned.

The CorDEX ToughPix II explosion-proof digital camera can be used in hazardous vapour areas without the risk of igniting the atmosphere, where a conventional digital camera would be a significant hazard.

Shrinking instrument size

Marcus Halliday, business development director at CorDEX Instruments argues that shrinking instrument size poses particular challenges for manufacturers in product development. 

The more buttons you need, the more openings you have and it becomes a lot more difficult, and sometimes impractical, to make an instrument explosion-proof, so you are then compelled to provide intrinsic safety,” he said.

Glands and cable connectors are relatively simple to manufacture and do not involve electronics, so EX d is relatively easy because you are using standard equipment, but with intrinsic safety you need to design from the ground up, which is what we and our engineers have come to learn over the last five years.

There are also many other different safety concepts like oil immersed for switchgear, or crystal immersed, both of which can be messy. If a device can be used in a hazardous area it does not necessarily mean it is intrinsically safe because an EX d explosion-proof device has the same level of certifications.”

Intrinsic safety devices are purpose built to stop potentially dangerous sparks or other damaging heating effects when working in volatile environments. These tools are essential for those who work in hazardous environments such as gas platforms, providing confirmation that the work environment will remain safe.

For example, the Fluke 725Ex intrinsically safe multi-function process calibrator has full certification for testing machinery in explosive atmospheres to ensure measurement accuracy is maintained.

It is also compatible with Fluke’s range of 700PEx pressure modules which are also made for use in explosive gas environments, which enable pressure measurements to 300psi/200 bar, allowing calibrating of pressure transmitters and I/P devices. A pressure switch test function is also included which allows capturing of sets, reset and deadband values.

Similarly, the Beamex MC2-IS Intrinsically Safe Calibrator connects to almost 20 available Beamex intrinsically safe pressure modules. It can be used in combination with various appliances and systems to measure pressure, current, voltage and frequency with built-in voltage, frequency and pulse generation plus mV measurement/simulation, resistance measurement/simulation, RTD measurement/simulation and TC measurement/simulation.

Conclusion

Intrinsic safety protects a device and its circuits in explosive environments, including the rupture, short circuiting or accidental grounding of its connecting wire. Qualified maintenance staff can test equipment even when the circuit is being powered and the plant is functioning.

When the hazardous location apparatus requires less than 30V and 100mA during fault conditions, intrinsic safety is the most effective, reliable and economical protection method.

PASS

www.pass.co.uk

T: 0845 365 4050

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