TechTopics No. 18

Bus joint and primary disconnect plating

ANSI/IEEE C37.20.2, the standard for medium-voltage, metal-clad switchgear, specifies temperature rise limits for buses and connections as shown in Table 1. From this table, it is clear that the manufacturer has a substantial incentive to use plated connection joints. Bus bars sized on the basis of unplated connection joints will require approximately double the material that will be required with silver-plated or tin-plated buses.

 

Why do the standards allow a higher temperature rise if the connecting joints are plated? The answer lies in an understanding of the basic objective of the design of a bus joint, which is to achieve a low-resistance joint that will remain so over the life of the equipment. Simple unplated copper-to-copper joints do not fulfill this objective, unless the temperature rise is held to a relatively low value. Copper oxide is a poorer conductor than pure copper, and copper oxidizes at a more rapid rate as temperature increases.

 

Silver plating is nearly ideal for switchgear bus joints. Silver corrodes (tarnishes) over time, but silver tarnish is nearly as good a conductor as pure silver. Silver is relatively hard, making it suitable for sliding joints (for example, primary disconnect assemblies) and for rotating joints (for example, moving switch blades). Perhaps the only disadvantage of silver is that it corrodes heavily in certain environments, particularly those having concentrations of hydrogen sulfide.

 

Tin plating is a good material for non-sliding and non-rotating applications. Tin maintains good conductivity over time, and performs well in the presence of hydrogen sulfide. However, tin is soft and has higher resistance than silver. Both of these characteristics make it unsuitable for use in primary disconnects for power circuit breakers or for moving switch blades.

 

Because of the overall performance advantage of silver, our practice is to provide silver plating for bus connection joints, primary disconnect assemblies, and cable connection terminal pads in switchgear and on circuit breakers.

Table 1: Temperature rise limits in switchgear
Application
Temperature rise limit
Buses and connections with unplated copper-to-copper connecting joints
30 °C
Buses and connections with silver-surfaced or tin-surfaced connecting joints
65 °C
Connection to insulated cables, unplated copper-to-copper (limits for connections to cables assume 90 °C cables)
30 °C
Connections to insulated cables, with silver-surfaced or tin-surfaced connections (limits for connections to cables assume 90 °C cables)
45 °C

As an option Siemens can provide tin plating of main bus connecting joints and cable connection terminal pads in the switchgear stationary structure. Primary disconnect assemblies and all plated connections on circuit breakers remain silver plated. Silver-plated surfaces should be coated with Siemens electrical contact lubricant to provide a degree of protection from harsh environments, such as those including hydrogen sulfide.

 

In order to maintain low-resistance connections over the life of the equipment, a few basic maintenance concepts should be observed:

  • Keep bus connections clean
  • Do not use abrasives (such as scouring pads, wire brushes, no-oxide grease) to clean plated surfaces
  • In all environments, keep sliding or moving surfaces lubricated with a thin film of Siemens electrical contact lubricant
  • In severe environments, keep all silver-plated surfaces lubricated with a thin film of Siemens electrical contact lubricant
  • Keep bolted joints tight.

Should you have any questions about this issue of TechTopics or any of our products, solutions, or services, please contact your local Siemens sales representative for more information.