Hello again!,
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In the previous post we learned about the insulated part of the terminal block/strip.Now its time to learn about the metallic parts of the terminal block.Basically in the last post we decided on the insulation of the terminal block,now for the ease of connection each wire needs to be connected separately and marked separately so that the engineer can properly identify it and he does not waste time in searching for the right connection.
You'll say that "In control cabinets space saving is our major concern!".That is definitely correct , but the width of the terminal blocks nowadays of taken very seriously.I can assure you that space saving would not be of your concern.Now getting back on the topic , we will be discussing about the current bar.
Before going into technicalities let's first try to understand why the current bar is used .Continuing from our previous post we have decided what kind of insulation housing our terminal block needs to have,now there are two ends in a terminal block from where the wire ends are terminated.Now those wires are terminated on the extreme ends of the terminal block and the body of the terminal block is non conducting so we need a conductor in between for the transfer of the current.So some conducting metal strip,could be used.Now a question arises that one of the problems that led to the invention of the terminal blocks was corrosion,so if we use metallic parts again wouldn't that lead to corrosion .
Before talking about corrosion let's talk about what kind of metal can be used for the current bar.Iron,Steel,copper,silver etc are some that come to the mind.All of them have their own set of advantages and disadvantages.Like silver is the best , but cost is too much high etc.So we go for copper (but even copper can get corroded??),so copper alloys are used in the making of the current bar.
Now that we have decided about what kind of metals we are gonna use.Let's see even copper alloys can succumb to the harsh environment.So we need an additional contingency plan to counteract corrosion effect.That naturally would be plating.
Now if plating ,then of what kind?,Which metal shall we use?,does that particular metal have any disadvantages?,will it protect the terminal block in all kinds of atmospheres? are some of the questions that pop into our mind.
Electrolytic copper or copper alloy current bar is usually coated with tin with a nickel undercoat.Now the nickel undercoat is due to the fact that if tin directly comes in contact with copper , then it reacts with it and zinc migration*effect takes place which is undesirable.
*In zinc migration the reaction produces zinc and this zinc gathers at one place thus neutralizing the effect of plating.
So ,basically the current carrying bar must have such a form that the conductor has a strong contact area in the connection point. Current bars are made out of brass / electrolytic cooper. Coating of tin/lead oxide, nickel while ensuring surface protection guarantees very low contact resistance.Low contact resistance is really a must property in the current bar as we always want maximum output.The thickness of the current bar varies according to its current carrying capacity i.e the more the capacity more thick the current bar.The Current bar is one of the two metallic parts in a terminal block.We'll take the other part in the next post.
There are many companies that make terminal blocks such as Phoenix contact,Connectwell,Weidmuller,Mouser,DigiKey,Allen Bradley,Schneider Electric,Altechcorp etc
Till the next time!
Keep asking why?!
Bye...
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