01 February 2005
CIS means high power intrinsic safety
By Udo Gerlach, Thomas Uehlken, and Ulrich Johannsmeyer
Contrary to the ripple-free direct current used in the industry so far, a safe power supply based on direct current superimposed by alternating quantities is also available. It is called the continuous interruption supply (CIS) principle. The CIS principle is part of a new intrinsic safety concept for achieving higher power in intrinsic safety circuits.
In the simplest case, this occurs by shorting a direct current source for short periods of time. While the conventional direct current practice admits power up to approximately 2 W (Division 1 and 2 Group B Hydrogen) for associated apparatus, the method of superimposition with accordingly defined alternating quantities enables an intrinsically safe effective power up to approximately 20 W. Thus, the intrinsic safety "i" type of protection can advance into power ranges that, so far, have included only more expensive types of protection, such as Flameproof Enclosure "d" IEC 60079-1 or Increased Safety "e" IEC 60079-7.
A broad-scale industrial implementation of this concept could significantly reduce the cost for manufacturers and users of explosion protected apparatus, and due to the typical advantages of intrinsic safety-worldwide acceptance, the possibility to work on energized systems, simple enclosure and connection techniques, and simple wiring requirements-the concept would also increase efficiency.
Explosion protection is a must for electrical apparatus used in potentially explosive atmospheres-large-scale chemical, petrochemical, or mining industries. Particular protection concepts, or types of protection, have been accepted internationally. Numerous international and national standards form the required legal framework conditions. One example is IEC 60079-11 for intrinsic safety "i." In accordance with this standard, an electrical circuit is intrinsically safe if it cannot, whether by a spark or by a thermal effect, cause the ignition of an explosive atmosphere during normal operation or under certain fault conditions. As a consequence of this standard, we can apply the intrinsic safety type of protection in the industry only for electrical circuits with a low power, mostly less than 2 W. Therefore its main field of application is currently only in measurement and control techniques. Compared to other types of protection, however, intrinsic safety also offers considerable advantages:
- Worldwide acceptance
- Simple requirements for enclosures (IP 20)
- Simple connection techniques for cables and lines
- Ability to work on the equipment- connecting and disconnecting or adjust- ing-without switching off the power supply
- Low manufacturing costs
These advantages weigh out against the current main disadvantage of intrinsic safety-the low effective-power output. Eliminating this disadvantage would give rise to new fields of application, so far confined to other more expensive types of protection.
With the new CIS concept, you can connect significantly more powerful intrinsically safe apparatus. On the basis of the CIS concept, you'll have future access to worldwide acknowledged, international examination certificates that permit a power output of up to approximately 20 W. By the industrial application of the CIS energy concept, you can also benefit from the advantages of the IS type of protection enumerated above in areas, where previously explosion protection was realized by more complex-and thus more expensive-types of protection, such as Flameproof Enclosure "d" IEC 60079-1 or Increased Safety "e" IEC 60079-7.
To achieve wider industrial acceptance of the CIS concept, not only energy transmission but also data transmission shall be possible by only one two-wire supply line.
Each periodical interruption of a direct voltage affects the emission of higher-frequency interference levels. To make this simple and uncomplicated, you can choose the data rate so the highest frequency required for data transmission is much lower than the repetition frequency of the CIS switching mode. It is possible to filter out the basic oscillation and its harmonics of the CIS switching mode by means of a simple low-pass filter. The pass band of this filter should only comprise the frequency ranges required for data transmission. Data transmission rates up to 50 kbit/s are realistic.
Behind the Byline
Udo Gerlach, Thomas Uehlken, and Ulrich Johannsmeyer are with Physikalisch-Technische Bundesanstalt Braunschweig Department of Explosion Protection of Electrical Equipment, Intrinsic Safety and Safety of Systems, Braunschweig, Germany.
CIS history lesson
The continuous interruption supply (CIS) concept comes from the fundamental investigations of the Physikalisch-Technische Bundesanstalt with sinusoidal alternating currents (a.c.) in the range from 50 to150 kHz. These investigations revealed, in comparison with direct current, a significant increase of the intrinsically safe effective power is possible. In this regard, extensive analytical descriptions of the electrical ignition parameters and their interactions are available.
However, a broad-scale industrial acceptance of this a.c. solution failed mainly as a result of line-depending limitations (mismatching) occurring when using alternating currents of higher frequencies. One of the most negative features was the limitation of the maximum connecting line length to below 30 m. In the meantime, investigations were made to find a feasible solution to eliminate this disadvantage. With the CIS concept, this has been successful.
Basic principles of CIS
The significant increase of the output power available with the use of alternating quantities instead of direct current comes from the periodical interruption of the electrical output for short periods of time. In the case of a.c., interruption occurs regularly in the zero crossings. From the point of view of ignition physics, no energy release into a possible spark can occur in the immediate vicinity of these ranges. Subsequent to this, a combustion zone in the process of emerging cools down, preventing the formation of an ignition-capable spark. Due to the permanent interruptions of the power transfer, breaking sparks, which usually cause an ignition (or are ignition-capable) with direct current (d.c.) power supplies, cannot develop at all.
The CIS concept takes advantage of this ignition-physical effect. However, sinusoidal a.c. quantities are not used here but define switching-mode d.c. quantities. With the use of d.c. quantities, it is possible to avoid the problems of the line caused from reflections due to mismatches occurring with the use of a.c. power supplies.