Why do grounding busbars (MGB / ground bar insulator kits) have insulators?

A

alej27

Member
Location
Venezuela
Occupation
Electrical Engineer
In commercial, industrial and telecommunication applications, it's common to find a ground bar insulator kit, commonly called master ground busbars (MGB), which are installed with a trapezoidal support and insulators, fixed in a wall.

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However, given that ground wires, metallic raceways, metallic boxes, cable trays, etc., are connected to earth ground and to building metal frames and rebars, why would we want to isolate the MGB from the wall?
 
There is no requirement to use the insulators. I guess that it allows you to use the ground bar isolated from the material it's attached to. You could remove the insulators if you like. Does anyone sell the kits with out insulators?
 
infinity said:
There is no requirement to use the insulators.
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Perhaps no NEC (as this forum applies) requirement, but at least the TIA 607-B does require it:
7 DESIGN REQUIREMENTS
7.2 Telecommunications main grounding busbar (TMGB)
7.2.1 General
The TMGB shall be as close as practical to the panelboard (electrical power panel) and shall be installed to maintain clearances required by applicable electrical codes. A practical location for the TMGB is to the side of the panelboard (where provided). The vertical location of the TMGB should be determined by considering whether the bonding conductors are routed in an access floor or overhead cable support. Its placement should provide for the shortest and straightest practical routing of the bonding conductor for telecommunications (BCT) and the primary protector grounding conductor (see annex C for more information on telecommunications electrical protection – primary protector grounding). Additionally, the TMGB shall be insulated from its support using an insulator that is Listed for the purpose by a nationally recognized testing laboratory (NRTL). A minimum of 50.8 mm (2 in) separation from the wall is recommended to allow access to the rear of the busbar.
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Does anyone sell the kits with out insulators?
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Not sure. Haven't seen it.
 
The MGB is isolated from the wall to prevent unintended parallel grounding paths that could introduce noise, stray currents, or ground loops. Even though everything eventually ties to earth ground, isolating the MGB ensures a controlled, low-impedance bonding point.
 
alej27 said:
In commercial, industrial and telecommunication applications, it's common to find a ground bar insulator kit, commonly called master ground busbars (MGB), which are installed with a trapezoidal support and insulators, fixed in a wall.

View attachment 2577746

However, given that ground wires, metallic raceways, metallic boxes, cable trays, etc., are connected to earth ground and to building metal frames and rebars, why would we want to isolate the MGB from the wall?
Click to expand...
Anybody know what Motorola R56 says about this?*
 
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