Basic Techniques of Intrinsically Safe Circuit Assessment

Sean L Clarke, Compliance Engineer, Epsilon Limited

Introduction

In order to make electrical equipment safe for use in a potentially explosive atmosphere, some form of protection must be afforded. Irrespective of whether you are certifying equipment for the ATEX Directive (for use in Europe) or to international requirements, the fundamental methodologies remain the same. One of the most commonly used concepts for low power equipment (such as instrumentation) is 'Intrinsic Safety'. This concept can be applied to achieve 'Zone 0' approval (or ATEX Category 1), where the potentially explosive atmosphere is continuously present. For this reason, the concept is considered to be one of the safest forms of protection, but also one of the most difficult concepts to apply.

Intrinsic safety relies on the electrical apparatus being designed so that it is unable to release sufficient energy, by either thermal or electrical means, to cause an ignition of a flammable gas. The energies required to ignite various gas groups have been proven by experimentation. Graphs of this data have been produced, and can be used to indicate safe levels of energy.

A very small amount of energy is required to cause an ignition, for example, a mixture of Hydrogen in air requires only 2OuJ of energy. In electrical circuits the mechanism for the release of this ignition energy is one or more of the following.

Open circuit or short circuit components or interconnections in a resistive circuit
Short circuit of components or interconnections in a capacitive circuit
Open circuit components or interconnections in an inductive circuit
Ignition by hot surfaces.

This article is a brief appraisal of the basic electrical considerations of power limiting to a safe level. The article is not concerned with many of the safety factors that are required during the design or certification process as it is intended as an overview to the subject. The safety factors, graphs and tables mentioned are references to British and European published standards (EN5O 020:1994 and EN5O 014:1992) for intrinsic safety.

Resistive intrinsically safe circuits

In a circuit that is non-reactive (quasi non-inductive/capacative) there is no stored energy to be released in an arc. The main consideration, therefore, is the amount of energy in the circuit. The power provided to intrinsically safe equipment is normally derived from batteries, a zener barrier or a galvanic isolator. Barriers are widely used in intrinsic safety to allow connection to power supply lines. They are used to limit both voltage and current (and hence power) into the hazardous area equipment. If a circuit can be proven to have no means of dissipating stored energy, and the voltage and current to it were proven to be safe, then the circuit will be safe electrically. (The thermal data of components would still need to be assessed so that ignition could not occur due to their heat in operation.)

A typical barrier is shown below.

In this form the shunt diode safety barrier will operate so that if a dangerous voltage is seen on the input, the diode (duplicated (i.b.) or triplicate (i.a) so that if one fails the barrier is still in operation) will hold the voltage down until the fuse open circuits. The resistor is added to reduce the short circuit current to less than that which is required to cause an ignition. The entire device is encapsulated so that if the fuse is stressed to the extent that it becomes open circuit, the whole barrier would require replacing.

Zener barriers such as this would be tested by a certification body and given output parameters such as: 28V
93.3mA
0.565W
(Derived from matched power V²)/4R

This simplifies the process of calculating the worst case power dissipation of the circuit that it is connected to.

Assessing the reactive circuit

As we are considering the safety of a circuit the most onerous conditions are taken. Stored energy in capacitors that could be released as a spark is an obvious problem (As the energy bi-passes the resistive safety protection of the supply). Two types of capacitance are considered for analysis, capacitance that can discharge without resistance and capacitance that can only discharge through resistance.

The worst case condition is if all the circuit capacitance was connected in parallel at worst case tolerance.

The total resistance added in parallel at worst case tolerance would give the least resistive protection. This will then give the largest capacitance with the lowest amount of resistive safety. A typical equated circuit is shown below.

Cl = All circuit capacitance not through resistance added (with tolerance added)

C2 = All circuit capacitance through resistance added (with tolerance added)

RT = All circuit resistance acting on C2 added in parallel (with tolerance subtracted)

These simplified values could then be checked against graphs of collated test data to see if there is a dangerous condition. Alternatively a simple circuit could be built that corresponds to the equated capacitance of the circuit under analysis. This circuit could then be tested on suitable test apparatus, known as 'spark test apparatus' or 'breakflash' apparatus. (This is process that involves the making and breaking of contacts in an explosive gas mixture, where the supply to the contacts is from the equivalent circuit.)

Inductive circuits are also calculated to give worst case conditions, which may involve the movement of any 'core' to simulate the highest possible inductance.

Other components that may effect the energy in a circuit can be tested individually. An example of this would be a piezoelectric device being subject to an impact test to record its output. (The 1/2CV² energy output from the device must be less than the energy required cause an ignition.)

Electrical Safety Seminars

EMV Ltd are planning a series of regional complementary one day seminars on the subject of Electrical Safety Testing, the first of which is scheduled to be run at Bletchley Park in September 2000.
The seminars, which are to be run in association with TRL and NEMKO will offer good background into achieving and maintaining product compliance and will review the requirements for production based testing.
The seminars are aimed at anyone who has responsibility or is involved with; the certification and compliance of their products; specifying and installing electrical safety test equipment; or setting up a "safe" production test station.
The topics which will be covered include:

Directives update;
Application & maintaining compliance for electrical safety;
Why test?;
The theory behind the 5 most common safety tests;
Production based testing;
Consideration for setting up a production safety test workstation.

For more information contact Mark Reeve, Tel: 01908 566556
Email: mark@emv.co.uk

Electrical Safety Seminars
EMV Ltd are planning a series of regional complementary one day seminars on the subject of Electrical Safety Testing, the first of which is scheduled to be run at Bletchley Park in September 2000. The seminars, which are to be run in association with TRL and NEMKO will offer good background into achieving and maintaining product compliance and will review the requirements for production based testing. The seminars are aimed at anyone who has responsibility or is involved with; the certification and compliance of their products; specifying and installing electrical safety test equipment; or setting up a "safe" production test station. The topics which will be covered include: Directives update; Application & maintaining compliance for electrical safety; Why test?; The theory behind the 5 most common safety tests; Production based testing; Consideration for setting up a production safety test workstation. For more information contact Mark Reeve, Tel: 01908 566556 Email: mark@emv.co.uk

Practical analysis

In a practical circuit it may be found that when all components are added in their worst case combinations the circuit is found to be ignition capable. A circuit can be made safe by employing various design techniques to limit the permutations of component connection. It is possible to design a circuit so that key safety components are considered to be infallible. Infallibility can be obtained theoretically for components by duplication and operation within 2/3 of their rating in worst case conditions. If three blocking diodes are linked in series then the voltage would be considered infallibly blocked and could be removed from a safety equation. When trying to isolate voltages or reactive components for analysis or design the interconnection of the various safety components must also be considered. This is referred to as physical analysis and can be used to give infallibility to interconnections. Creepage and clearance requirements are used to reduce the risk of interconnections becoming open or short circuit. The protection afforded to interconnections from short circuit conditions is insulation. An interconnection is protected from an open circuit by its physical strength. If a printed circuit board track is designed such that it has a width of 2mm and is correctly insulated (for example by conformal coating) it can be considered infallible.

The importance of infallible tracking is shown above . The high capacitance is required for circuit operation, but is found to be unsafe at 24V. Circuit operation is not affected if the capacitor is clamped to 12V, so a zener clamp is employed. If the track from the capacitor to the clamp was not infallible, it would be considered as open circuit. This would render the clamp across the capacitor inoperative and 24v would be seen. Creepage and clearance distances and components operating under their rated values are highly salient in intrinsic safety analysis. Depending on the category of intrinsic safety required (which is dependent on the type of classification of the potentially explosive atmosphere it will be employed in) a circuit can be considered with 'countable faults'. If a component is proven to be operating within 2/3 of its ratings in worst case conditions, its failure can be considered countable. Countable faults can be used to alleviate the requirement to apply safety factors to many calculations. If a circuit is being analysed with two countable faults, for example, it may be shown that if two correctly rated components failed a large voltage could be seen across a capacitor. The component would only require safety analysis at that voltage when normally the voltage would be multiplied by a factor of 1.5. Component interconnections can be considered countable if they are greater than or equal to specified table values.

Types of protection

There are various types of protection and power limitation components that can be used in intrinsic safety. All components that are considered necessary to maintain the safe operation of a circuit (the safety components) require special consideration.

Zener diodes must be used within 2/3 of all their specified ratings. Creepage and clearance distances would have to comply with table values and duplicated diodes would be employed.
Diodes are used when blocking is required, they are often in series with the output of part of a circuit that is potted. They must be used within 2/3 of their ratings and be either duplicated for redundancy.
Resistors would be used to dissipate power and limit current. They are often employed to limit the instantaneous release of charge from capacitors. They would be required to meet a similar criterion to the zener diodes but would also have constructional requirements.
Capacitors can be used as safety components for blocking DC voltages, but must be duplicated in series. Electrolytic and tantalum capacitors can not be used, and capacitors that are used are required to pass specific high voltage tests.
Fuses must be potted if it is used in a hazardous area. It must have a breaking capacity of 1500A (so as not to conduct after breaking) if it is used to protect a main's transformer. It limits power only and is not used to limit instantaneous current.
Transformers and opto-couplers can isolate intrinsically safe from non-intrinsically safe circuits. They are subject to creepage and clearance requirements and special tests, such as high voltage break-over tests on transformers.
Semiconductor clamps can be employed in some types of intrinsic safety, but require careful analysis.

The safety components are used in various configurations to limit voltage, current and power to safe levels. For clarity, the methods of employment and operating conditions of the various components listed are a simplification of the analysis that would be required to ensure a circuit was safe.

Connections to an intrinsically safe circuit

When calculating the maximum safe values of reactive components consideration must be given to the capacitive and inductive values of any cable that may be required to connect to the circuit. A value of capacitance and inductance must be stated as a maximum allowable connection value to the intrinsically safe circuit. As a safe level of reactance will have been established in the circuit, a value of capacitance that is considered suitable for the operational use of the circuit can be added to that value. This new figure can then be tested for safety by use the methods previously discussed. If the breakflash apparatus is used to establish the safety of the new value of reactance, silver mica capacitors can be used to simulate the cable capacitance since they behave in a similar manner to cable. The inductive value of the cable may also take into account the resistance of the cable (L/R ratio) and its effect on the matched power.

Temperature requirements of an intrinsically safe circuit

As a component with a high surface temperature could cause an ignition, the maximum temperature that any component on a circuit board under fault conditions must be considered. The temperature that a component can rise to can be related to its thermal resistance (when its power is limited by a safety barrier). The temperature of a component related to the power input can be obtained from thermal data sheets. Alternatively, tests can be carried out on ten samples to find a representative temperature rating. Smaller components will need to reach a higher temperature than the auto ignition temperature of a gas to cause an ignition. (For the simplification of temperature assessment, EN 50020:1994 states that small components from 20mm2 to 10cm2, that can draw a maximum power of 1.3W can be said to have a maximum temperature of 200°C. If the component is less than 20mm2 then it can be given a maximum temperature rating of 275°C.)

Summary

Intrinsic safety is dependent on a circuit operating with low power and acceptable temperatures so that it does not have the required energy to ignite a flammable atmosphere. This can be obtained for most types of circuit at a relatively low cost if certain design techniques are employed. When designing or certifying an intrinsically safe circuit, all possible scenarios for connections of components are considered (unless they can be proven infallible) and many safety factors have to be included when calculating the safe circuit parameters. It is this combination of factors that allows intrinsically safe electrical equipment be sited in the most hazardous areas.

Sean Clarke is a compliance consultant with Epsilon Technical Services Limited. Epsilon offer compliance design consultancy, testing and certification for equipment intended for use in potentially explosive atmospheres. Epsilon also offer a complete CE Marking service, and environmental / performance testing.

Contact: s.clarke@epsilon-ltd.com; Telephone: 01244 541551 or Fax: 01244 543888

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