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:

dmulligan@weidmuller.co.za

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