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Electrical Standards |
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Manufacturers have a responsibility to ensure that their products
are safe for their intended use, however they cannot control the
application itself. Therefore it is important that end users
understand the background and purpose of the electrical standards
which have been developed for their protection.
The standards are complex and voluminous. Few people not involved in
their application will find the time or interest to familiarise
themselves with their content, however they are of great importance
to end users. This general guide is written to provide an insight
into the standards and how they are applied.
They generally follow the recommendations of the INTERNATIONAL
ELECTROTECHNICAL COMMISSION [IEC] and are issued by:
- CENELEC ( Comite
Europeen de Normalisation ELECtrotechnique) in
Europe where they are mandatory by law
- NEMA (National Electrical
Manufacturers Association) in the USA where they
have effectively similar requirements under US legislation.
In addition to the CENELEC and NEMA
standards some countries, including the United Kingdom, issue
national standards related to IEC recommendations which have not
been adopted as yet by these major bodies. One such important
standard is BS 6981 : 1988 for non-incendive apparatus and which
takes account of IEC 79-15:1987.
Hytork® Controls manufacture and supply electrical products, switch
boxes, positioners and solenoid operated control valves which comply
with applicable standards.
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What The Standards
Cover
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Standards for use in potentially explosive atmospheres are generally
based on the IEC 79 SERIES "ELECTRICAL APPARATUS FOR EXPLOSIVE
ATMOSPHERES".
The CENELEC and NEMA standards precisely define the limiting
conditions and environments for the use of such apparatus.
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CENELEC Standards in the EN 50
Series are limited to "Electrical apparatus for use in potentially
explosive atmospheres."
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NEMA 250 series covers both
hazardous areas (potentially explosive atmospheres) and
non-hazardous areas.
The CENELEC EN 50 Series is confined to
specifications for use in potentially explosive atmospheres. The
general specification is EN 50 014.
NEMA 250 standards covers both hazardous and non-hazardous
applications.
These standards are expanded by a series of special specifications
covering various conditions of use and the methods.
by which they are met. Some of these standards are approximately
equivalent and where applicable both CENELEC and NEMA standards are
covered by the HYTORK® designs.
STANDARDS FOR USE IN POTENTIALLY EXPLOSIVE (HAZARDOUS)
ATMOSPHERES
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CENELEC |
| TECHNIQUE |
SYMBOL |
SPECIFICATION |
IEC ZONE |
| GENERAL |
|
EN 50 014 |
| OIL IMMERSION |
'o' |
EN 50 015 |
2 |
| PRESSURISATION |
'p' |
EN 50 016 |
1 and 2 |
| POWDER FILLING |
'q' |
EN 50 017 |
2 |
| FLAME PROOFING |
'd' |
EN 50 018 |
1 and 2 |
| INCREASED SAFETY |
'e' |
EN 50 019 |
1 and 2 |
| NON-INCENDIVE (**see note below) |
'N' |
BS 6941 (** EN 50 021) |
2 |
| ENCAPSULATION |
'm' |
EN 50 028 |
1 and 2 |
| SPECIAL PROTECTION |
's' |
Specific designs. |
0, 1, and 2 |
| INTRINSIC SAFETY |
'ia'' |
EN 50 020 |
0, 1, and 2 |
| INTRINSIC SAFETY |
'ib' |
EN 50 039 |
1 and 2 |
| HERMETIC SEALING |
'h' |
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1 and |
NOTE:
NON-INCENDIVE "N". This standard takes
account of the recommendations given in IEC 79-15 :1987 and EN 50
021 has been tentatively allocated but has not yet been issued. It
is covered in the UK by BS 6941: 1988.
This standard is used in areas where protection is required against
potential fire or explosive atmosphere hazards in Zone 2 locations,
and covers an extremely wide range of applications. The ambient
temperature range is limited to -20 to + 40 degrees C.
NEMA.
HAZARDOUS (CLASSIFIED) ENCLOSURES.
TYPE 7 ENCLOSURES FOR INDOOR USE IN CLASS 1, GROUPS A,B,C,D TYPE 8
ENCLOSURES FOR INDOOR AND OUTDOOR USE IN CLASS 1 GROUPS A,B,C,D.-
OIL FILLED TO CONTAIN ARCING. TYPE 9 ENCLOSURES FOR INDOOR USE IN
CLASS 11, GROUPS E,F,G. THESE ENCLOSURES ARE SPECIFICALLY DESIGNED
AGAINST DUST/AIR MIXTURES. TYPE 10 ENCLOSURES FOR USE IN MINES.
The similarities between some of the CENELEC AND NEMA specifications
are exemplified by the definitions :-
CENELEC FLAMEPROOF "d".
Flameproof "d" is defined as --- " a type of protection in which
parts which can ignite an explosive atmosphere are placed in an
enclosure which can withstand the pressure developed during an
internal explosion of an explosive mixture and which prevents the
transmission of the explosion to the explosive atmosphere
surrounding the enclosure.
NEMA TYPE 7 is defined as - "Enclosures which shall be capable of
withstanding the pressures resulting from an internal explosion of
specified gases and which contain such an explosion sufficiently
that an explosive gas/air mixture existing in the atmosphere
surrounding the enclosure will not be ignited. Enclosed heat
generating devices shall not cause external surfaces to reach
temperatures capable of igniting explosive gas/air mixtures in the
surrounding atmosphere".
STANDARDS FOR NON HAZARDOUS LOCATIONS.
NEMA also issues standards for non hazardous indoor and outdoor
location.
TYPE OF ENCLOSURE
| PROTECTION AGAINST
CONDITIONS OF USE. |
1 |
2 |
3 |
3R |
3S |
4 |
4X |
5 |
6 |
6P |
11 |
12 |
12K |
13 |
| INDOOR USE. |
| INCIDENTAL CONTACT WITH
ENCLOSED EQUIPMENT, |
X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
| FALLING DIRT. |
X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
| FALLING LIQUIDS. |
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X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
X |
| FLYINGS |
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X |
X |
X |
X |
X |
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X |
X |
X |
| HOSE DOWN |
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X |
X |
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X |
X |
| OIL, COOLANT SEEPAGE |
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X |
X |
X |
| OIL, COOLANT SPRAYS |
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X |
| CORROSIVE AGENTS |
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X |
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X |
X |
| OCCASIONAL IMMERSION
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X |
X |
| PROLONGED IMMERSION
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X |
| OUTDOOR USE. |
| RAIN, SNOW OR SLEET
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X |
X |
X |
X |
X |
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X |
X |
| SLEET |
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X |
| WINDBLOWN DUST |
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X |
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X |
X |
X |
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X |
X |
| HOSEDOWN |
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X |
X |
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X |
X |
| CORROSIVE AGENTS |
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X |
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X |
| TEMPORARY SUBMERSION |
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X |
X |
| PROLONGED SUBMERSION |
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X |
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Physical Protection of electrical Apparatus |
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The enclosures or containments of electrical apparatus have to
satisfy certain criteria to give protection from dust, liquids,
people and impact and details are given in ---
IEC 529:1976. CLASSIFICATION OF DEGREES OF PROTECTION PROVIDED BY
ENCLOSURES
These are characterised by IP followed by a three digit
number.
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IP- means Ingress
Protection
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The first digit signifies the
degree of protection against solid objects, hands, fingers, probes
and debris. Zero means no protection and rising numbers indicate
increasing protection against diminishing size of objects up to 6
which gives total protection against dust.
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The second digit shows the degree
of protection against liquids. Zero means no protection rising from
drips through sprays of varying directions and intensities up to
8 which gives protection against total immersion for long
periods.
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The third digit ( which is not
always used ) shows the built in protection against impact damage.
Zero means no protection. Impact energies are specified in joule and
range from 1 which equals 0.225 joule (150 g dropped
from 150 mm ) to 9 which equals 20 joule ( 5kg dropped from
400mm)
For example IP 465 means that it is protected against solid
objects over 1mm (4), strong jets of water from all
directions (6 and impacts up to 2.00 Joule-500 g dropped from 400mm.
(5)
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Testing & Certification |
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Control and maintenance of the standards is maintained in the
participating countries by accredited test houses which are
assessed for competence by national control bodies.
There are many organisations such as BASEEFA and SIRA in the UK---
UL and FM in the USA--- PTB and BVS in Germany---- IERIS and LCIE in
France and so on.
These Test Houses conduct independent tests on products according to
the schedules laid down in the appropriate standard. Certification
of compliance includes design and manufacturing specifications,
sealed designs, manufacturing methods or processes and a Quality
Assurance requirement for assessment and surveillance.
The Test Houses have their own distinguishing mark to be applied to
products which they have certificated.
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Marking Of Approved Apparatus |
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Marking of approved apparatus must comply with the standard against
which it was made. This must be on the product in such a manner as
to prevent its removal. For example for CENELEC it will have
the manufacturer, the type of product, the symbol EEx signifying
compliance with a CENELEC standard, the symbol for the type of
protection, the gas group and sub-division, the temperature
class,usually a serial number and the Test House mark and date of
test.
Apparatus tested to standards other than CENELEC, e.g. BS 6941 would
carry the symbol 'Ex N'. For example a hypothetical apparatus
made by BLIGGS certificated to the standard EN 50 018 Flameproofing
'd', by Test House "H" for use with Group II gases subdivision B, at
a maximum temperature rating of 135 degrees C. would be marked EEx -
d - IIB - 135C (T4). It would carry the Test House mark "H" 94
if performed in 1994. It would carry the company Trademark
"BL" and a serial number for Q A tracability.
For NEMA the marking would include the Manufacturers name, the NEMA
Type of enclosure, the Class and Groups with which it may be used,
the temperature rating, the test house and certificate number and a
Q A tracablity serial number. Typically this could be ---
"B" Serial no 1234 - "H" Certificate No 5678.- NEMA
Type 7 Class 1 Group C - 165 C (T3B).
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Hazardous
Zones |
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The IEC recognise three Zones where explosive mixtures of gas or
powder and air can exist.
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Zone O Where
the mixture is continuously present.
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Zone 1 Where the
mixture is likely to occur in normal operations.
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Zone 2 Where a
mixture is unlikely or will only exist for a short time.
Zones, such as, are not
mentioned in either CENELEC or NEMA standards.
However they are characterised in the U K Code of Practice BS
5345:Part1:1989.for the application of the CENELEC standards.
NEMA approximate the IEC Zones into divisions, Zones 0 and 1 are in
Division 1 and Zone 2 is Division 2.
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What Are The
Hazards?
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The hazards are the
explosive mixtures which can consist of flammable gases or
combustible dusts mixed with air. Different gases
and dusts are characterised into groups according to the danger
level. The tables below shows the increasing danger of the
groupings. The CENELEC and NEMA ratings are approximately the same.
| EC / CENELEC |
NEMA |
| GROUP 1 |
CLASS 1 GROUP D |
| GROUP 11A |
CLASS 1 GROUP D |
| GROUP 11B
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CLASS1 GROUP C |
| GROUP 11C
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CLASS 1 GROUPS A and
B
NEMA also list Combustible Dusts and Flyings. |
| CLASS 11 : GROUP E |
METAL DUSTS, MAGNESIUM,
ALUMINUM, ZIRCONIUM, ETC. |
| CLASS 11 : GROUP F |
CARBON BLACK, COAL AND COKE
DUSTS ETC. |
| CLASS 11 ; GROUP G
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FLOUR, STARCH AND GRAIN
DUSTS. |
| CLASS 111 : |
FLYINGS, COTTON LINTERS,
SAWDUST ETC,.. |
These potentially explosive mixtures
may be ignited by naked flames or sparks, by hot surfaces and by
spontaneous ignition if the mixture temperature reaches a certain
level.
The relevant temperatures leading to ignition are classified under
"T" numbers based on an ambient temperature of 40 degrees C. The
NEMA specifications further subdivide the "T" numbers as shown
in the table below.
| CLASS |
MAXIMUM TEMPERATURE |
NEMA TEMPERATURE
SUB DIVISIONS |
| T1 |
450 C |
| T2 |
300 C |
T2A -280 C, T2B -260
C, T2C- 230 C, T2D- 215 C. |
| T3 |
200 C |
T3A -180 C, T3B- 165
C, T3C- 160 C. |
| T4 |
135 C |
T4A- 120 C. |
| T5 |
100 C |
| T6 |
85 C |
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