TERMINATION TECHNOLOGIES FOR HAZARDOUS AREAS A practical guide

Transcription

1 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; 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:

2 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 tinplated 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 3.3 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 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.

4 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 6mm 2 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. 5. CABLE AND WIRE PREPERATION Figure10 Types of conductors 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 600 o 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.

5 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

6 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.

7 Figure 16 Typical Crimping faults 6. TERMINAL STANDARDS AND TESTS Two standards form the basis for assessing conductor connections being: IEC (VDE ) and IEC (VDE ). IEC is the general rules for low voltage switchgear and control gear and IEC 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 0.2mm 2 and 300mm 2, 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.

8 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 Mechanical tests The following tests must be performed for proof of mechanical features Mechanical strength of the terminal (VDE Section ) 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.

9 Figure 18 Mechanical strength of terminal Proof of secure connection in the terminal point. A secure connection is proven by two tests being: The bending test (IEC section ) and the mechanical load test (IEC 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 Proof of rated cross section. (IEC section ) 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) b b a a Form A Form B Figure 21 Proof of rated Cross section

10 6.2 Type tests according to IEC Electrical tests The following tests must be performed for proof of electrical features Voltage Drop test (IEC 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 Temperature rise test (IEC 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 45 o C during the test. Again the volt drop test must be performed before and after this test. C Check point Figure 23 Temperature rise test layout

11 6.2.3 Thermal short-circuit withstand test (IEC 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/mm 2 ) is applied for at least one second. Again the volt-drop test must be performed before and after this test to verify conformance. MCheck e ß ppoints u n k t Figure 24 Thermal Short-circuit withstand test layout Ageing test (IEC 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 20 o c. The terminals are subject to 200 temperature cycles up to 120 o C 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.

12 Direction of vibration Figure 25 Vibration Resistance test 7.2 Natural resonance behaviour (IEC ) 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. Conductor secured Vibration table Oscillation direction in 3 axis Figure 26 Natural resonance test

13 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 -65 Test according to IEC Products subject to temperature 65 C Duration 2 days Voltage drop Correct function Visual appearance 100 C Dry heat test according to IEC i Products are subject to dry heat of +130 C. Duration: 7 days Voltage drop Correct function Visual appearance 93% Damp heat constant test according to IEC 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 C Damp heat cyclical test according to IEC Products are subject to cyclical temperature changes at high humidity as follows: +40 C at a relative humidity of 93% +25 C at a relative humidity of 97%. Duration: 10 days Voltage drop Correct function Visual appearance SO 2 H 2 S Sulphur Dioxide Test according to IEC 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 SO 2 at +25 C and 75% humidity Storage for C Duration: 10 days Hydrogen Sulphide Test according to IEC 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 H 2 S at +25 C and 75% humidity Storage for C Duration: 10 days Voltage drop Correct function Visual appearance Voltage drop Correct function Visual appearance

14 Salt fog Test according to IEC 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 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

15 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 130 o C 165 o C 100 o C 120 o C Temperature Tracking Resistance CTI 600 CTI 600 CTI 600 CTI 600 (CTI) Dielectric strength 10kV/mm 16kV/mm 30kV/mm 25kV/mm 10.1 Insulation material tests Flammability test (UL-94 V0 V2) Table 2 Overview of Insulating materials for terminals 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. 300 mm 127 mm Figure 30 Flammability Tests

16 Glow wire test EN /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 550 o C to 960 o C 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 960 o C, 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 /02/2008/SMMD [2] Weidmuller Technical Guide. Hazardous Areas Technical Guide, Part no /2011/SMMD