Grounding 4160 Switch Gear

[chadwiseman]

We have multiple 46/4160 transformers in our substation that feed switchgear inside our facility. The power cables are teck, and run in multiple cable trays. In a recent inspection we noticed that there was no bare ground to each switchgear (they are old installations). Should we have seperate bare ground conductors to each switchgear from each power transformer? When we have multiple power cables in a tray would be need to bond both bare cables? If one power transformer feeds multiple switchgears, does the bare ground need to come directly from the transformer ground to each one, or can you run one and tap off to each switchgear?

Thanks.

 

[alehman]

Are you sure there is no ground? If the cable tray is metallic, continuous and installed properly, it can be used as the equipment ground in most cases. Most engineers, myself included, would not choose to do that. The ground does not have to be bare. It may be 600V insulated type or it may be within the jacket of multi-core cable.

If in fact there is no equipment ground, you should probably install one. The ground can be tapped if it is sized adequately for the largest upstream overcurrent protection. In the case of multiple cables, if they are from different sources, it must connect to all sources. That can create protection problems. I would recommend careful analysis of the installation by a qualified engineer before proceeding.

 

[PWR]

The transformers and switchgears may well be properly grounded by connection to an underground grid. Quite frankly "they are old installations" usually means a pretty conservative approach was utilized for grounding. If the 4160V system is resistance grounded, there is less concern that a high ampacity grounding conductor be run in close proximity to the phase conductors.

 

[jghrist]

Even if both the transformers and switchgears are connected to an underground grid, there needs to be an equipment grounding conductor run with the phase conductors. This is to ensure a low impedance for a ground fault path. If the ground return path is distant from the phase conductors, the reactance will be high.