TERMINATION TECHNOLOGIES FOR HAZARDOUS AREAS – A practical guide
D.G Mulligan Pr Eng*
* Managing Director, Phambili Interface (Pty) Ltd, P O Box 193, Edenvale, 1610, South Africa; e-mail:
ABSTRACT
The purpose of this document is to outline the various termination technologies used for various wire connections in
terminals on switchgear and field junction boxes in normal and hazardous areas. In industrial applications there are five
main types of terminal termination technologies being screw clamp, tension clamp, push in, stud and Insulation
Displacement Connections (IDC). This document will highlight the principle and function of each termination
technology. This document will also discuss correct cable and wire preparation for any type of termination. This
document will also highlight the various mechanical, electrical and climatic or environmental tests performed on
terminal connectors in accordance with the IEC standards and other international standards. The document is written to
assist design engineers and end users into the basic aspects for the selection of particular termination technologies for
their specific applications.
1. INTRODUCTION
Increasing functionality, compact designs as well as a high degree of complexity of devices and systems in power
supply and control technology are placing high demands on the electrical connections and connection technology.
Choosing the right connection system is imperative to ensure a safe, reliable and maintenance friendly system or
electrical installation. Each termination technology has its unique benefits and pitfalls and in selecting the correct
termination technology at the beginning ensures long-term reliability and safety of electrical apparatus and plants.
When one selects the relevant technology or terminal for a particular application it is an advantage to understand the
factors and environment the terminations will be required to perform in. Quality products will all have international
independent test authority approvals and certificates outlining all tests carried out on terminations. Obtaining these will
assure the user that the products have been tested and comply with the minimum standards required for any application
or installation.
2. FACTORS TO CONSIDER FOR TERMINATIONS
The following factors and conditions must be assessed prior to selection over and beyond normal rated termination
specifications for current and voltage. These factors include:
Area of operation: Are the terminals in an enclosed environment with no air impurities or hazardous corrosive gases or
dusts?
Hazardous gases and dusts: Will the terminals be used in a hazardous area and what is the de-rating current applicable
for the application?
Space limitations: Is there space limitations in the panel or junction boxes?
Time to assemble: Are there time constraints to assemble the panel or junction box having a direct impact on labour
costs?
Type of conductors to be terminated: Are the conductors stranded or solid? Do they require a ferrule on each
conductor end?
Vibrations: Will the terminals be subject to severe vibrations?
3. CONNECTION TECHNOLOGIES
There are various connection technologies of which the most common are discussed below.
3.1 Screw Clamp Connection
This type of connection is illustrated in Figure 1 and consists of the following:
A clamping yoke normally made of galvanised zinc-plated and chrome passivated case hardened steel, together with a
high-strength clamping screw, which holds the conductor securely and reliably in the terminal point. A separate tin-
plated copper busbar (current bar), which provides good contact with low contact resistance. A connection of this type
allows separation of the electrical and mechanical functions being a clamping yoke and screw made of steel for
optimum contact force and a busbar made of copper to ensure good electrical connection and minimize voltage drops.
A side view of this connection is given in Figure 2 and illustrates that a high contact force is achieved by tightening the
screw to the correct torque, which causes the flank of the thread to open slightly and lock the screw in position. This
ensures a vibration resistant connection. With a screw clamp connection it is permitted to connect several conductors
at one terminal point due to the U type design of the clamping yoke.
Figure 1 Screw Clamp Connection Figure 2 Vibration free Screw Clamp Connection
3.2 Tension Clamp Connection
This type of connection is illustrated in Figure 3 and consists of a tension clamp normally made of high-grade
corrosion-resistant steel and a separate tin-plated copper busbar (current bar). There is again separation of the electrical
and mechanical functions. The tension clamp is used to ensure connection between conductor and current bar with a
good contact force. No reliance is made on tightening a screw for connection and this technology quicker than screw
clamp connections. For tension clamp technology it is only permitted to connect one conductor per terminal point,
however if a dual ferrule is used, two conductors could be connected in one terminal point as shown in figure 4 below
Figure 3 Tension Clamp Connection Figure 4 Two conductors in a dual ferule
3.3 Push In Connection
This type of connection is illustrated in Figure 5 and consists of a tension spring normally made of high-grade
corrosion-resistant stainless steel and a separate tin-plated copper busbar (current bar). There is again separation of the
electrical and mechanical functions. The tension spring is normally at a 30- 40 0 angle and up against the current bar
without any wires inserted. Wires either solid or with a ferrule can then be pushed into the terminal until they reach the
bottom point and the tension spring will hold the conductor firmly in the middle against the current bar ensuring a 100%
vibration free connection between conductor and current bar with an excellent contact force. Because of the angle of
the tension spring in relation to the conductor, these terminations cannot be pulled out and to release the conductor a
screwdriver is required to release the spring. No reliance is made on torque tightening a screw for connection. For push
in technology it is only permitted to connect one conductor per terminal point.
Figure 5 Push In Connection
3.4 Insulation Displacement Connection (IDC)
This type of connection is illustrated in Figure 6 and consists of a cutting element and an external spring. The cutting
element is made of high-grade copper alloy with a pure tin surface and the external spring is made of high-grade
corrosion-resistant steel. In this type of connection, no preparation of the conductor to be terminated is required and a
simple mechanism as illustrated in Figure 7 shows a slide action that cuts the insulation of the conductor and this is
pushed into the external spring allowing the bare conductor to connect to the copper contact (current bar) while the
external spring ensures good contact force. Installation time using this technology is greatly reduced (up to 75%),
however only one conductor (solid or stranded) per terminal point is allowed.
Figure 6 Insulation Displacement Connection Figure 7 Operation of IDC type connections
3.5 Stud type Connection
This type of connection is illustrated in Figure 8 and consists of a threaded stud inserted through a thick tined copper
current bar between the studs. As per Figure 9 tin plated copper cable or wire ring lugs of the correct size are inserted
over the studs and torque tightened with a serrated washer and metric nut made of high-grade corrosion-resistant steel.
Once terminated a cover and partition separates each termination or cable from the others. These types of connections
are normally preferred for high current applications (Greater than 100 amps) and ensure an extremely effective and tight
connection.
Figure 8 Stud Terminals Figure 9 Ring lug connection on stud terminal
4. CONDUCTORS FOR TERMINATIONS
Most terminals should be suitable for connecting solid, stranded and flexible conductors as shown in Figure 8 below.
Should a ferrule be fitted to flexible conductors to prevent individual wires splitting apart, these ferrules must be
crimped to the conductor. The rated connecting capacity of the terminal is normally stated by the manufacturer and
must be adhered to. For hazardous areas one must take into account the de-rating of terminal currents and voltages
especially if terminal cross connectors are being used. These de-ratings will be specified in the relevant Ex certificates
from the manufacturer. The rated connecting capacity is the range of wire sizes that can be accommodated per
termination point. For example a 6mm2 terminal can accommodate a conductor from 2.5mm
2 up to 6mm
2 only.
Use of aluminium conductors is also permitted, but preparation of aluminium conductors prior to termination is
required to remove oxide coatings and a grease coating applied to protect the aluminium once connected.
Figure10 Types of conductors
5. CABLE AND WIRE PREPERATION
Correct preparation of conductors and cables is the most critical process for any termination. One can select the most
suitable termination technology for the application, but an inferior cable or wire termination (either via poor preparation
or use of the incorrect ferrule or lug) can lead to a poor connection with arcing and severe heat developing on the
termination point. Poor connections and arcing can lead to temperatures in excess of 600o C and this can lead to
breakdown and worst case fires and explosions. The three critical operations for cable or wire preparation are as follows:
1. CUTTING of the cable or conductors
2. STRIPPING of the PVC or other type of Insulation
3. CRIMPING of ferrules or lugs
MAKE SURE YOU USE the correct tool for all three of these vital areas
5.1 Cutting
Cutting is the severing of copper or aluminium cables or conductors using an appropriate cutting tool. The minimum
requirement is a smooth straight cut without distortion of the conductors as shown in Figure 11 below. DIN 8588
specifies the requirements for shear cutting of cables and conductors with minimum force with tools that are shaped to
conform with conductors or cables.
Figure 11 Clean cuts versus dirty cut of cables and conductors
Figure 12 shows examples of correct cutting tools that conform to the DIN standard
Figure 12 Cutting tools for wire and cable conductors
5.2 Conductor Stripping
Stripping is the severing and removal of cable or conductor insulation. When doing cable or conductor stripping it is
important that none of the current carrying conductors are damaged or removed in any way during the stripping process
or the remaining insulation is not damaged. DIN IEC 352 Part 2 contains references to various stripping faults that
could occur and may have an effect on the overall performance of a connection as shown in Figure 13 below.
Figure 13 Stripping faults
Figure 14 shows examples of different stripping tools that conform to this standard
Figure 14 Stripping Tools
5.3 Crimping
After conductors or cables have been stripped it may be desirable to install an appropriate lug or wire end ferrules over
the bare conductors which are crimped to the cable conductors or wires. These lugs or ferrules are normally made of
tinned copper and many have a plastic collar that goes over the conductor insulation for protection.
Crimping produces a reliable and gas tight connection between conductors and the lug or ferrule in place of soldering.
The result is a safe, reliable electrical and mechanical connection. The user can use different crimp types and shapes to
provide the optimum connection, but the correct size lug or ferrule that is suitable or optimal for the size of conductor
must be used as shown in figure 15. Never use a lug or ferrule that is much larger than the conductors as this will
distort or crack the lug or ferrule when crimped. Always use a crimper that has a full ratchet stroke (Must complete the
crimp before releasing) and make sure the lug or ferrule is inserted in the correct slot for the applicable size on the
crimp required. Always ensure that no strands of conductor protrude more than 0.5mm outside the lug or ferrule end.
This can be assured by setting the correct stripping distance on the stripping tool.
Figure 15 Optimal Ferrule crimps for wire or conductor cross section
Outlined in Figure 16 below are typical faults that could occur with crimping of ferrules or lugs. It is recommended to
batch check for some of these faults during factory acceptance tests prior to commissioning or testing apparatus.
Figure 16 Typical Crimping faults
6. TERMINAL STANDARDS AND TESTS
Two standards form the basis for assessing conductor connections being:
IEC 60947-1 (VDE 0660-100) and IEC 60947-7-1 (VDE 0611-1). IEC 60947-1 is the general rules for low voltage
switchgear and control gear and IEC 60947-7-1 being Part 7 which defines the standards for ancillary equipment and
section 1 which is specific for terminal blocks for copper conductors. The scope of section 1 specifies the requirements
for terminal blocks with screw or screw less type connections intended to connect round copper conductors, with or
without special preparation having a cross section between 0.2mm2 and 300mm2, intended for use in circuits of rated
voltage not exceeding 1000Va.c. up to 1000Hz or 1500V d.c.
These standards contain all essential regulations according to which the connections are to be designed, produced and
tested to ensure they can perform their required tasks at all times under difficult industrial conditions. These standards
also form the basis for qualitative assessment of terminal points contained within terminals. Some manufacturers carry
out further type tests over and above the ones prescribed in the IEC standards in order to conform to certain national
approvals or specific (environmental) conditions.
In figure 17 below is an example of all tests performed by a certain manufacturer.
Figure 17 Contact safety tests performed by Weidmuller
For the plastic terminal insulated housings there are two main standards being:
1. The flammability classification in accordance with UL-94 and
2. The comparative tracking index classification or CTI (IEC 60112)
6.1 Type tests according to IEC 60947-7-1 – Mechanical tests
The following tests must be performed for proof of mechanical features
6.1.1 Mechanical strength of the terminal (VDE 0660 – 100 Section 8.2.4.2)
In this test, the stipulated type of conductor must be used and the largest connectable cross section must be used. The
conductor must be connected and disconnected five times to the required test torques as per Figure 18 below.
Figure 18 Mechanical strength of terminal
6.1.2 Proof of secure connection in the terminal point.
A secure connection is proven by two tests being:
The bending test (IEC 60947-7-1 section 8.2.2.1) and the mechanical load test (IEC 60947-7-1 section 8.2.2).
In the bending test the terminated conductor is loaded and moved around over 135 times in a suitable test apparatus as
shown in Figure 19. The conductor must not slide out of the connection or break in the vicinity of the terminal.
In the mechanical load test a suitable test appartus as shown in figure 20 must be used and the conductor must be loaded
with a defined tensile force for one minute continousely. This pulling force should be increased to the point where the
conductor is pulled out and this must be noted. (Normally 2 – 3 times the required pulling force is excellent)
Figure 19 Flexion (Bending) test apparatus Figure 20 Mechanical conductor load test apparatus
6.1.3 Proof of rated cross section. (IEC 60947-7-1 section 8.2.2.3)
This test is carried out with set calibrated gauges as shown in Figure 21. The measuring pin must able to be inserted
and reach the end position of the connection opening purely by means of the weight of the gauge only (not exceeding 5
N)
Figure 21 Proof of rated Cross section
b
a
Form A Form B
a
bb
a
Form A Form B
a
b
6.2 Type tests according to IEC 60947-7-1 – Electrical tests
The following tests must be performed for proof of electrical features
6.2.1 Voltage Drop test (IEC 60947-7-1 section 8.3.2)
The voltage drop test is a standard test and must be performed before and after the mechanical tests described above, the
temperature rise test, the short-time current withstand test and the ageing test (These tests are described further on in
this document)
For the voltage drop test the volts across the terminal termination points is measured at rated current as shown in Figure
22 below. A maximum limit of 3.2 mV volt drop is allowed.
Figure 22 Volt drop test
6.2.2 Temperature rise test (IEC 60947-7-1 section 8.3.3)
The temperature rise test is carried out with five terminals mounted side by side, connected in series with insulated
conductors of the rated cross section as shown in Figure 23 below. A constant rated single-phase A.C. current is passed
through the terminals and the temperature rise is measured at the centre terminal until a constant temperature is
recorded. This temperature in the middle should not rise by more than 45oC during the test. Again the volt drop test
must be performed before and after this test.
Figure 23 Temperature rise test layout
Check point
C°
Check point
C°
6.2.3 Thermal short-circuit withstand test (IEC 60947-7-1 section 8.3.4)
The purpose of this test is to prove the terminal can withstand the thermal shock triggered by a short-circuit. For this
test a single terminal is wired with a stranded conductor of the largest rated connectable cross-section as shown in
Figure 24 below. The rated short circuit current (equivalent to a current density of 120A/mm2) is applied for at least
one second. Again the volt-drop test must be performed before and after this test to verify conformance.
Figure 24 Thermal Short-circuit withstand test layout
6.2.4 Ageing test (IEC 60947-7-1 section 8.3.5)
This test is normally only conducted on tension clamp and IDC terminals and follows the temperature rise test. This
test requires five terminals that have successfully been subjected to the voltage drop test are place in a heating cabinet at
a starting temperature of 20oc. The terminals are subject to 200 temperature cycles up to 120
oC with each cycle lasting
approximately one hour each. The volt drop is measured after every 25 cycles up to the 200 cycles have been
completed. Again the limit on the volt drop is 3.2mV.
7. OTHER CONNECTOR TESTS
Each manufacturer has the right to perform other tests depending on the relevant application and specific customer
requirements or local/national standards applicable. Some of these tests are briefly described below
7.1 Vibration Resistance Test (DIN 57611/VDE 0611 part 1)
This test is carried out with the terminal largest and smallest rated conductor cross-section. The conductor is secured to
the required test torque and a mechanical load is connected to the other end of the conductor as per Figure 25 below.
The terminal is then subject to vibrations for 2 hours with a frequency = 12 Hz and amplitude of 1mm and the same
with a frequency of 50 Hz. The test piece is then rotated 90 degrees and the test is repeated to register the effect of
oscillation on the terminal in an altered position. The criteria for test success is that the conductor does not slide out,
break in the vicinity of the terminal point and that the voltage drop after the test does not exceed 150% of the starting
value.
M e ß p u n k tM e ß p u n k tCheck Points
Figure 25 Vibration Resistance test
7.2 Natural resonance behaviour (IEC 60068-2-6)
This test determines the resistance of the components and devices to sinusoidal oscillations and the set up is as per
figure 26 with the conductors bent and secured. Oscillations of between 10 and 500Hz are applied and again the criteria
for test success is that the conductor does not slide out, break in the vicinity of the terminal point and that the voltage
drop after the test does not exceed 150% of the starting value.
In addition to this tests for resistance to shock due to demands on sea-faring vessels can also be performed. These tests
are vibration tests according to BV 0440 for surface ships and a shock test according to BV 0430 for surface ships and
submarines.
Figure 26 Natural resonance test
Direction of vibration
Conductor secured
Vibration table
Oscillation direction in 3 axis
8. CLIMATIC INFLUENCES AND TESTS
Electrical components are exposed to vastly differing environmental conditions all over the world and it is imperative
that devices and systems function flawlessly despite climatic influences. Such climatic conditions include:
• Temperature
• Relative Humidity
• Condensation
• Chemicals
• Air quality
Various standardised test have been developed to tests termination devices under varying conditions and these are
summarised in the Table 1.
CLIMATE CONDITION TEST METHOD RESULTS CHECK
Test according to IEC 60068-2-1
Products subject to temperature – 65°C
Duration 2 days
Voltage drop
Correct function
Visual appearance
Dry heat test according to IEC 512- 6-11i
Products are subject to dry heat of +130°C.
Duration: 7 days
Voltage drop
Correct function
Visual appearance
Damp heat constant test according to IEC
60512-6 test 11 c
Products are subject to constant temp of +40°C
at a relative humidity of 93%.
Duration: 10 days
Voltage drop
Correct function
Visual appearance
Damp heat cyclical test according to IEC 60068
–2-30
Products are subject to cyclical temperature
changes at high humidity as follows:
12hours @ +40°C at a relative humidity of 93%
12hours @ +25°C at a relative humidity of 97%.
Duration: 10 days
Voltage drop
Correct function
Visual appearance
Sulphur Dioxide Test according to IEC 60068-2-
42. Test is used to assess corrosive effect of gas
on contact surfaces of noble metal and
impermeability and function of electrical
connections
Products are subject to 10ppm concentrated SO2
at +25°C and 75% humidity
Storage for 48 hours @ 80°C
Duration: 10 days
Voltage drop
Correct function
Visual appearance
Hydrogen Sulphide Test according to IEC
60068-2-43. Test is used to assess corrosive
effect of gas as an element in contaminated air
contact surfaces of electrical connections
Products are subject to 1ppm concentrated H2 S
at +25°C and 75% humidity
Storage for 48 hours @ 80°C
Duration: 10 days
Voltage drop
Correct function
Visual appearance
-65°����
C°
100°
C°
100°
C°50°
25°
SO2
H2S
93%
Salt fog Test according to IEC 60068-2-11 to
assess the resistance to salt fog
Products are subject to salt fog NaCl 50g/l at
35°C
Duration 2 days
Voltage drop
Correct function
Visual appearance
Table 1 Climatic and environmental tests
9. GAS TIGHT TEST (IEC 60512-6 test 11n)
This is a visual test for the terminals subject to the last 3 tests mentioned in section 8 above where each connection must
prove to have gas tight areas covering at least 75% of the positions where the terminal current bar comes into contact
with the conductor. The gas tight areas are lighter and sharper in contrast to the areas that have discoloured as a result
of storage in the test atmosphere. The results for the 3 types of connection technologies are shown in Figure 27, 28 and
29 below.
Figure 27 Gas tightness on screw clamp connection
Figure 28 Gas tightness on tension clamp connection
Figure 29 Gas tightness on IDC connection
10. TERMINAL HOUSING INSULATION MATERIALS AND TESTS
There are basically two types of insulating materials being thermosetting plastics and thermoplastics. Each type has
unique characteristics and properties and in comparison thermosetting plastics such as melamine have outstanding
dimensional ability, low water absorption, excellent creepage resistance and a very high fire resistance. However
thermosetting plastics are less flexible mechanically than thermoplastics. In table 2 below is a basic overview and
characteristics of insulation materials used for terminal housings.
THERMOSETTING PLASICS THERMOPLASTICS
Germin KrG Epoxy Resin EP Polyamide PA 66 Wemid
Flammability
classification (UL-94)
V-0
V-0
V-2
V-0
Mech. Properties rigid rigid flexible flexible
Cont Operating
Temperature
130oC 165oC 100oC 120oC
Tracking Resistance
(CTI)
CTI 600 CTI 600 CTI 600 CTI 600
Dielectric strength 10kV/mm 16kV/mm 30kV/mm 25kV/mm
Table 2 Overview of Insulating materials for terminals
10.1 Insulation material tests
10.1.1 Flammability test (UL-94 V0 – V2)
In this test, an open flame of 20mm is exposed to the material sample for two periods of 10 seconds each, as shown in
figure 30 below. The pass criteria for V0 classification is the sample must self extinguish, have a post burning time less
than 10 seconds and any drops on the lint below must not burn. The pass criteria for V2 classification is the sample
must self extinguish, have a post burning time less than 30 seconds and any drops on the lint below can burn.
Figure 30 Flammability Tests
127 mm
300 mm
10.1.2 Glow wire test EN 60695-2-1/1)
In this test a hot glowing wire is pushed into the side of the insulating material for a period of 30 seconds in steps from
550oC to 960
oC as shown in figure 31 below. The criteria for passing this test is that the post burning time must be less
than 30 seconds after glow wire touches and there must be no burning drops. Both Wemid and KrG (Melamine) are
classified for 960oC, but PA 66 (Polyamide) does not pass this test.
Figure 31 Glow wire test
11. REFERENCES
[1] Weidmuller Product Information. Termination Technology – Product Information, Part no
5661520000/02/2008/SMMD
[2] Weidmuller Technical Guide. Hazardous Areas Technical Guide, Part no 1261090000/2011/SMMD