Wiring Harness News Sep-Oct 2022 - Page 8

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8 SEPTEMBER / OCTOBER 2022 Wiring Harness News INDUSTRIAL INFO-TAINMENT

High Voltage Cable Processing

Continued from page 6 _____________
Once you get rid of the outer jacket , you need to remove the shield . First comes the foil which is basically a metal coated plastic tape . That has to be done very cleanly because you can ’ t have any of that inside the connection area between the ferrule and the braid . Once you remove the foil , you need to cut the copper braid itself . This must also be done very cleanly . Also , the braid has to be flared so that you can crimp the ferrule over it before you crimp the terminal to the inner conductor .
Finally , you have to strip the inner jacket . Like the outer jacket , you have to be very careful that you cut deep enough so you ’ re not tearing the insulation , but not too deep to risk

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nicking the inner conductor , which could cause issues to the electrical characteristics once you ’ ve completed the crimp .
WHN What are the crimp challenges with large cables ?
Martin : Obviously the main difference is the force needed to crimp wires of this size . With low-voltage wire , you ’ re typically crimping at less than 3000 lbs . With high voltage cables , you ’ ll be crimping around about 10,000 lbs . for midsize wire ( upwards of 6 gauge ) and then you can be as high as 40,000 pounds for the larger cables like the 90 and 120 mm ². I would say most high-voltage terminations at the moment are in
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Figure 3 . Example of “ clocking .”
the 10,000 and 20,000 lb . range in terms of crimp force .
The next difference compared to small wire sizes is what we call “ clocking ”. It ’ s very important to know exactly where the center line of a conductor is because , if you can imagine , it ’ s almost impossible to twist a high-voltage cable . For long leads , you have some give in the cable . But for shorter leads and asymmetric terminals , the terminals will need to be installed in the proper orientation compared to the terminal on the other end of the cable ( Figure 3 ) & this is what we mean by “ clocking ” the conductor .
The third difference to look for with crimping high-voltage cables is extrusion . Extrusion happens when you crimp any wire size , but obviously the larger the conductor crosssection , the more significant effect . Just to give you an idea , a 70 mm ² wire will extrude about 5 mm . So , it ’ s very important to take that into consideration when you are calculating exactly where the wire needs to be inserted prior to crimping . Again , this is more important for shorter cable assembly lengths .
The last issue I want to talk about with respect to crimping , is cracking . Cracking can occur when crimping very large size wire terminals . It ’ s particularly unpleasant because you usually can ’ t see it with the naked eye . The only way you can detect it with a cross section analysis , so you really want to avoid any risk of cracking .
Usually cracking happens for three reasons . First , it can happen if you over crimp the wire . That ’ s why it ’ s very important to follow the manufacturer ’ s recommendation
for the crimp height . It can also happen if you are using inappropriate wire sizes . So , for example , if you ’ re using a wire which is too big for the terminal , you might get cracking . Finally , and most important , it can also occur if the crimp cycle is too fast . We have found that in crimping very large wire sizes , that speed is very important . That ’ s why TE , and other high-quality crimp presses , have adjustable speeds . Typically , the larger the wire size the slower the optimal crimp speed .
In rounding out the discussion of crimping , I should mention that ultrasonic welding is also being used quite a bit in high-voltage terminal connections . With ultrasonic welding , you avoid some of the challenges that I ’ ve just described with crimping . However , ultrasonic welding is a costly capital outlay . It ’ s also slower than crimping and it requires very high levels of energy . It ’ s definitely an option , but I would say the majority of high-voltage connections today are still made with the crimping process .
The last area where we have a significant differential compared to small wires is the handling . That ’ s quite simply due to the sheer weight of these high-voltage cables . Just to give you an idea , A 50 mm ² cable that is only 2 m long weighs 1.5 kg . That really does limit how you manipulate these cables in the assembly area . You can ’ t form large bundles of wire as you might with small wire sizes . It would just be inconceivable to move such a weighty assembly . This also limits the extent to which highvoltage cable production can be automated .
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