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Intrinsically Safe - Type of protection to EN 60079...

 

Introduction

 

"Intrinsically Safe" (IS) is a protection certification for safe operation with electronic equipment in explosive atmospheres. The concept was developed for safety in explosive hazardous areas like petrochemical plants, where explosive hydrocarbons compounds, hydrogen and ammonia are common industrial gases. The concept extends to other areas with explosive hazards, including utilities, marine transfer operations, granaries, pharmaceuticals, consumer packaged goods, alcohol processing, paint manufacturing and many others.

A device termed "intrinsically safe" is engineered, designed and, most importantly, certified to be incapable of producing heat or spark sufficient to ignite and explosive atmosphere, regardles of the source: flammable gasses, vapors, or combustible dusts or fibers. Tablets, cell phones, smart phones, PDAs and other hand-held devices can be ignition sources for explosives. An intrinsically safe device will not ignite explosives.

As a basic principle, in the same way as for all other types of protection, the associated electrical apparatus used in an intrinsically safe circuit and the intrinsically safe apparatus have to be tested and certified. According to EN 60079, only apparatus where, according to the details given by the manufacturer, none of the values exceed 1.2 V; 0,1A; 20µJ or 25 mW are exempted.

Simple intrinsically safe apparatus, where the electrical data and the thermal behaviour can be clearly defined and that conform to the applicable constructional requirements, do not need to be tested or certified.

This applies, for example, to:

  • switches
  • plugs and sockets
  • terminal boxes
  • measuring resistors
  • individual semi-conductor components
  • coils (moving coil instruments)
  • capacitors
  • electric position sensors (DIN 19 234)

However, the general requirements according to EN 60079-0 and the design of enclosures and connection boxes with regard to the surface resistance or the choice of aluminium alloy (surface resistance of plastic enclosures <109 Ohm) shall also be observed here.

Limiting ignition curves.

The energy set free in an intrinsically safe circuit in the event of a failure must be limited to such a degree that the occurence of an ignition is prevented. To faciliate matters, the limiting ignition curves for the individual groups were established with the aid of a standardized spark test apparatus.

As the probability of the ignition of a mixture also depends on the number of switching operations, according to EN 60079-11 at least 1000 switching operations shall be performed, whereby an ignition must not occur under any circumstances. Depending on the design of the intrinsically safe circuit, the existence of stored energy has to be taken into account. If there are capacitances in the intrinsically safe circuit, these are added on to the voltage of the circuit. In the event of a short circuit, in addition to the energy supplied by the associated apparatus the energy stored in the capacitor is released. The same applies if there are inductances in the circuit. For these reasons all three borderline cases, i.e. ohmic, capacitive and inductive circuits have to be taken into consideration.

Under here you'll see diagram represented:

Ohmic circuits

Ohmic circuits

Minimum ignition current to be applied for electrical apparatus containing cadmium, zinc, magnesium or aluminium.

Capacitive circuits

Capacitive circuits

Minimum ignition voltages to be applied to group IIC electrical apparatus. The curve marked Sn is only to be applied to electrical apparatus not containing any cadmium, zinc, magnesium or aluminium.

Inductive circuits

Inductive circuits

Minimum ignition current to be applied for electrical apparatus containing cadmium, zinc, magnesium or aluminimum at U = 24 V.

All images can be clicked on for a large image.


Level of protection

Intrinsically safe apparatus and intrinsically safe parts of associated electrical apparatus are allocated to a level of protection "ia", "ib" or "ic".

Level of protection "ia"

With Um and Ui applied, the intrinsically safe circuits in electrical apparatus of level of protection "ia" shall not be capable of causing ignition in any of the following circumstances:

  1. in normal operation and with the application of those non-countable faults which give the most onerous condition;
  2. in normal operation and with the application of one countable fault plus those non-countable faults which give the most onerous condition;
  3. in normal operation and with the application of two countable faults plus those uncountable faults which give the most onerous condition.

The non-countable faults applied may differ in each of the above circumstances.

In testing or assessing the circuit for spark ignition, the following safety factors shal be applied;

  • 1.5 for both point 1. and 2. and 1.0 for point 3.

The safety factor applied to voltage or current for the determination of the temperature class in accordance with the surface temperature hall be 1.0 in all cases.

If only one countable fault occur, the requirements of point 2. are sufficient for the level of protection "ia" if the test requirements for "ia" can then be satisfied. If no countable faults occur, the requirements of point 1. shall be considered to give a level of protection of "ia" if the test requirements for "ia" can then be satisfied.

Level of protection "ib"

With Um and Ui applied, the intrinsically safe circuits in electrical apparatus of level of protection "ib" shall not be capable of causing ignition in any of the following circumstances:

  1. in normal operation and with the application of those non-countable faults which give the most onerous condition;
  2. in normal operation and with the application of one countable fault plus those non-countable faults which give the most onerous condition.

The non-countable faults applied may differ in each of the above circumstances.

When testing or assessing the circuits for spark ignition, a safety factor of 1.5 shall be applied. The safety factor applied to the voltage or current for the determination of surface temperature classification shall be 1.0 in all cases.

If no countable fault can occur the requirements of point 1. shall be considered to give a level of protection of "ib" if the test reqirements for "ib" can then be satisfied.

Level of protection "ic"

With Um and Ui applied, the intrinsically safe circuits in electrical apparatus of level of protection "ic" shall not be capable of causing ignition in normal operation.

In testing or assessing the circuits for spark ignition, a safety factor of 1.0 shall be applied. The safety factor applied to the voltage or current for the determination of surface temperature classification shall be 1.0 in all cases.

NOTE:

The concept of countable faults does not apply to this level (ic) of protection.


Apparatus with galvanic isolation

Transformers that, for example, are built into intrinsically safe power supplies shall ensure a safe galvanic isolation between the primary and the secondary circuit.

The data that is relevant for the intrinsically safe circuit is determined by the output characteristic of this transformer and, if available, of any external circuits (voltage limitation by diodes, current limitation by resistors or by the electronics).

Isolation of intrinsically safe circuits form non-intrinsically safe circuits

In order to avoid any mistakes when connecting or bridging conductors, the terminations of the intrinsically safe circuits shall be safely isolated from the terminations of a non-intrinsically safe circuit.

For this purpose, the terminals of the intrinsically safe circuit can, for example, be installed at a distance of at least 50mm. from the terminals of each non-intrinsically safe circuit or they can be separated by an isolating partition wall or an earthed metal partition wall. These partition walls shall either reach up to 1.5mm. from the enclosure wall or they shall ensure a minimum clearance of 50mm. between the connection terminals.

Isolation of insulated leads of intrinsically safe circuits and non-intrinsically safe circuits

The distance between the conductors of insulated cables shall satisfy the given requirements. With the exception of varnishes and similar coatings, this insulation is considered to be a solid insulation.

The clearances are determined by the addition of the radial thickness of the insulation on the wires. The minimum clearances are laid down in EN 60079-11:

e.g.

for U ≤ 60 V.   0.5mm.

for U ≤ 750 V. 1.4mm.

The voltage U is the sum of the voltages of the intrinsically safe circuits or the intrinsically safe and non-intrinsically safe circuits. This clearance is not required, if:

  • the cores of either the intrinsically safe or the non-intrinsically circuit are enclosed in an earthed screen, or -
  • in level of protection "ib" electrical apparatus, the insulation of the intrinsically safe cores is capable of withstanding an r.m.s. a.c. test voltage of 2000 V.

Moreover, it is necessary to ensure that no inductive or capacitive interferences from the non-intrinsically safe circuit can occur in the intrinsically safe circuit.

Planning of intrinsically safe circuits

When setting up an intrinsically safe circuit with just one intrinsically safe apparatus and one associated apparatus, the limiting values for the permissible external capacitance and the permissible external inductance shall be taken from the type label of the associated apparatus. They are decisive for the circuit. Then, based on the maximum possible power supply of the associated apparatus, it is only necessary to check the heat rise behaviour of the intrinsically safe apparatus. However, when several intrinsically safe circuits are interconnected (e.g. several pieces of associated apparatus for one intrinsically safe apparatus), a further verification of the intrinsic safety is necessary. The verification of the intrinsic safety of a circuit shall be documented in detail.

Intrinsically safe circuits with Zener barriers

Normally intrinsically safe circuits shall be insulated. They may be earthed if this is required for functional reasons. They must, however, be earthed if this is mandatory for safety reasons. This earth connection may only be made at one point by connection with the potential equalization, which has to be available throughout the area in which the intrinsically safe circuit is installed. As, in the case of safety barriers, there is no galvanic isolation of the intrinsically safe circuit from the non-intrinsically safe circuit, for safety reasons there has to be a faultless earth connection.

Working on and testing of intrinsically safe circuits

On principle, working on live intrinsically safe circuits is allowed, as, due to their design, no ignition can take place. However, the generally valid requirements for work on live parts shall be observed. When using measureing instruments, it is necessary to bear in mind that they might contain an internal energy store (e.g. the inductance of a moving coil indicator), that could nullify the intrinsic safety.

Reduced separation distances

Under certain conditions specified in the standard, it is possible to design printed circuit boards with seperation distances that are smaller than the standard distances that normally apply, thus enabling the use of smaller, more effective electronic elements.

4-wire PT_100_-_example This is examples of an intrinsically safe application: 4-wire PT 100

Click on the image to enlarge it.


Refences:

  • Standard: EN 60079-0 Explosive atmospheres - Part 0: Equipment - General requirements (IEC 60079-0:2007)
  • Standard: EN 60079-11 Explosive atmospheres - Part 11: Equipment protection by intrinsic safety "i" (IEC 60079-11:2006)
  • Inspiration and some information from: Eaton/Crouse-Hinds/Ceag and Aegex Technologies, CorDEX Instruments.

It is the user's responsibility to ascertain if a particular product is safe and without risk to health and safety by virtue of its location in a hazardous area, i.e. classification of zones, gas groups, ignition temperatures, etc. Both the specifier and user should be thoroughly familiar with standard mentioned on this sites or any document within.

Whilst every care has been taken in the compilation of this document, we regrets that it cannot accept responsibility for any errors or omissions contained herein. Readers should not rely upon the information contained in this document or site without seeking specific safety advice and ensuring that their own particular circumstances are in accordance with the matters set out.

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