How to Limit The Current and Voltage in Intrinsically Safe (IS) Hardware Design?
Intrinsically safe (IS) hardware design stands at the forefront of ensuring safety in hazardous environments. The fundamental concept behind intrinsic safety is to meticulously control voltage and current, preventing any possibility of sparks with the potential to ignite explosive atmospheres. IS circuits are ingeniously engineered to operate seamlessly under normal conditions, but more crucially, to keep energy levels well below the ignition thresholds in the event of a fault.
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In this comprehensive guide, we delve into the critical aspects of IS hardware design, illuminating the design criteria, cost-saving strategies, and an innovative solution, the PPEX Power Protection Module, developed by ADOTT Solutions. Whether you’re an engineer, a safety professional, or a technology enthusiast, understanding how to limit current and voltage in IS design is pivotal for ensuring safety in high-risk environments. Let’s embark on this journey to explore the core principles of IS hardware design and the tools that empower it.
What is Intrinsic Safety (IS)?
Intrinsic safety is the method of protection for control. The concept of intrinsic safety is to limit the voltage and current so that there is never a spark with enough energy to create an explosion. Intrinsically safe circuits are designed so that they operate properly under normal conditions, but keep the energy levels below the ignition curves when a fault condition occurs.
What is Design Criteria for the IS Circuits?
Intrinsic safety is an explosion-prevention design technique suited to instrumentation circuits and solid-state technology. Electronic process control and telemetering systems – using relatively low voltage and low power – that place all or part of the circuits within a hazardous location take advantage of IS.
The criterion used in analyzing an intrinsically safe system verifies that the maximum thermal and electrical energies released in a hazardous location are much less than the Minimum Ignition Energy (MIE) of the specific atmosphere, both in normal and abnormal or fault conditions.
The procedure calculates the maximum open-circuit voltage Voc and the utmost short-circuit current Isc allowed in the hazardous location.
When selecting apparatuses for the hazardous location, it is crucial to consider the ignition temperature – which varies with the gas characteristics – plus the ignition energy because they do not correlate. Note that gas may suddenly ignite when heated above its ignition temperature. Consequently, decide on the temperature class for the application.
How to Reduce Design Cost and Certification Cost?
The equipment in the safe area may pose a hazard by delivering unsafe energy levels to the hazardous location. Installing suitable power transfer limiters between the non-hazardous and hazardous locations may reduce this risk.
This electrical barrier ensures that the energy entering the hazardous location – arriving from the non-hazardous location – cannot come close to the level at which ignition occurs. The barrier is installed in a non-hazardous location or inside an explosion-proof enclosure in a hazardous location.
These challenges are solved by the protection module developed and certified Ex Component by ADOTT Solutions.
Ex Component: It's intended to be part of Ex Equipment, marked with the symbol “U”, which is not intended to be used alone, and requires additional consideration when incorporated into explosive protected equipment.
The A.PPEX module is an intrinsically safe electronic barrier for power lines in certified intrinsically safe electrical equipment or associated apparatus. It can be used where high power and low loss must meet intrinsic safety. The A.PPEX module has an over-current, and an over-voltage protection. There are several variants of the unit, which allows the developer to select the appropriate current limit between 0.7 to 3.3 A and the voltage limit between 5.7 and 12.6 V.
PPEX – Power Protection Module
It is useful in battery-powered applications for hazardous environments, where a circuit might have to be shut down immediately to avoid a dangerous situation due to a short circuit or other fault.
Specific applications include:
- Oil- and gas transportation, exploration
- Offshore rigs and pipelines
- Mining applications, such as drilling and tunneling
- Agricultural uses, such as grain elevators
What Does the Power Protection Module Do?
This module is ensuring a limited current on its output, and in specific variants it will also limit the voltage on its output. However, for application of the module, further consideration is required to ensure that the combination of voltages, currents and components (e.g. capacitors, inductors) that are to be connected to the output of the module, will maintain the safety of the equipment (e.g. via Annex A of IEC 60079-11 or spark assessment).
Block Diagram
The Over-Current Protection logic and the Series MOSFET Switch realize the fast-acting current limiter. This part of the circuit provides a constant current output in case of an over-current event. When the current limit has to act for a longer time period, the Control Circuit switches off the Series MOSFET Switch to protect the circuit elements from overheating. After the recovery time has elapsed Control Circuit retries to switch the output on again. The Under-Voltage Lock-Out (UVLO) circuit disables the unit and switches off the output when the input voltage is under the healthy voltage level. The Disable input can be used for switching the output off. The optional Over-Voltage Protection (Crowbar) shorts the output of the unit when the output voltage exceeds a certain tripping voltage level. The short circuit activates the Over-Current Protection that acts as it is described previously.
Current Limit Timing Diagram
Before tOC the PPEX module is conducting the IN current. This is the normal operational mode. When an over-current event occurs over the value of IL at tOC a current spike runs through on the power line. The time of this spike is the reaction time of PPEX current limit circuitry that is typically between 100-200 ns. After this time period, the circuitry realizes the over-current event and terminates it by switching the output off. The energy of the spike is under 20 uJ. After 10-100 us the unit enters constant current mode for 3-5 ms and switches off the output for 350-500 ms. The module tries to switch its output on periodically while it continuously provides current limitations.
How to use the Power protecion module?
Check out the details on the product page and get in touch with our team to answer any further questions.
