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Why Would I Use a NAMUR Output Sensor?
Posted by Robert Pasho on Thu, May 30, 2013
Proximity sensors and encoders can have NAMUR output
NAMUR is a type of sensor output that gives an on or off
indication. There are different reasons why you may need a NAMUR
sensor. I will go through
some of them here.
First, some information about NAMUR. NAMUR is a German acronym
for (normenarbeitsgemeinschaft fr Mess-
und Regeltechnik in der Chemischen Industrie). This translates
to: Standardization Association for Measurement
and Control in Chemical Industries. A NAMUR sensor is a
non-amplified sensor that supplies two different signal
levels depending upon switch state. These signal levels are
low-level current. A logic interface circuit,
controller, PLC, DCS, or intrinsically safe (IS) barrier is
needed to read these current levels. A NAMUR output
sensor is required in installations where special safety
measures are necessary (locations with explosion hazard,
or personal safety). The standards for NAMUR output sensors are
EN 60947-5-6:2000 and IEC 60947-5-6:1999.
A NAMUR output sensor can be normally open or normally closed,
but usually, it's normally closed. This output is
solid state, so there will be some current level on the output
whether its on or off. When you look at a NAMUR
output sensor data sheet, it will list nominal voltage (8.2 V),
load (1 k ohm), and current for measuring plate
detected (2.2 mA) (for normally closed output). Because of the
low-current switching
level, you'll need an amplifier or an input that is designed
specifically for NAMUR.
Although the sensor may beable to handle a range of voltages (7
VDC to 12 VDC or 5 VDC to 25 VDC), the voltage level should be at
8.2 VDC
with 1k ohm resistance as the load.
Proximity sensors and encoders can have NAMUR output. Some of
the specific types of proximity sensors that have NAMUR output are
capacitive,
inductive, magnetic, and photoelectric sensors. Depending on the
application, you could need any one of these products to detect
your target, but if
they meet the NAMUR standards, the output would be the same.
The NAMUR output products that have Ex ia (intrinsic safety)
approvals can be installed in hazardous (classified) locations,
such as Class I, II, III;
Division 1, 2; and Zone 0, Zone 1, Zone 2 areas, as long as an
approved intrinsically safe (IS) input or barrier is used with the
NAMUR output product.
The Zonal protection is usually for European and Class, Division
is for North America. Some approval types are ATEX, FM, UL, IEC,
CENELEC, TUV,
PTB, and CSA.
Prevention is the protection principle for intrinsic safety. It
limits current so that the current is not capable of causing
ignition in hazardous
atmospheres. The other protection methods are containment
(explosion-proof or flame-proof enclosure) and segregation
(purging, pressurization,
encapsulation, or oil filled). Using intrinsic safety has its
advantages: there are no special cables required or expensive
explosion-proof conduit; it's
safe for personnel and suitable for all area classifications
(Class I, II, III, Division 1, 2 and Zone 0, 1, 2).
If personal safety is needed, there are versions of the NAMUR
output that are also fail-safe and have a SIL3 rating (with the
appropriate amplifier). A
fail-safe output ensures that if there is a failure, the output
will sense a problem and fail only in the safe mode. With this
NAMUR fail-safe output,
you will also need the fail-safe-rated input or amplifier. This
amplifier could be dual rated for personal safety and classified
hazardous locations for
gas, dust, or fibers. With this fail-safe sensor and amplifier,
this system would have SIL3.
Usually, NAMUR output products (which include encoders and
proximity sensors) are used in classified hazardous location areas
which have explosive
gas, dust, or fibers present. The fail-safe NAMUR output sensor
is used where there is a personal safety hazard to protect
personnel when they are
working at or near the equipment. A fail-safe NAMURversion with
the appropriate intrinsically/fail-safe barrier can provide both
explosion hazard and
personal safety protection.
Tags:Photoelectric Sensors, Inductive Sensors, Rotary Encoders,
Terminology, Capacitive Sensors, Approvals/ Certifications,
Magnetic Sensors
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