CVT circuits cable shield grounding

[marks1080]

I am revisiting some of the grounding techniques we use on CVT circuits.

With respect to the CVT secondary cables... Generally, I would want to ground the cable shield at both ends of the cable (good for high freq. transient suppression). I also know that the bonding conductor connecting the shield to ground should be as short as practically possible. Reducing the length of the bonding conductor will reduce its impedance - an important characteristic when dealing with very high freq. transients (lightning or disconnect switches drawing arcs for example).

So what happens if I want to use two bonding conductors? If these are both now in parallel between the cable shield and ground, the total impedance of the bonding conductors to ground must now be smaller than using just a single conductor. If this is true, could using parallel bonding conductors on cable shields be a means of improving surge suppression?

Thoughts? Comments?

Thanks,
Mark

 

[collies99]

These are low voltage circuits, 67, 69, 115 or 120v secondary.
Typically wired with either armor sheathed cable or shielded cable, jackeyed but without armor.

I do think there are special requirements with regards to high frequency transients on these cables/conductors.

On the teck cable, the armor is bonded to the structure and the termination box through the connector and the cable ground is grounded in the box. On the shielded cable, the shield is cut inside the box and special jaw clamp interfaces with the shield and a grounding wire in the box. Gounding in madr to the ground terminsl in thr box.

The bottom of the CVT structure is grounded externally to the ground grid.

The other end of the cable may or may not be grounded, utility have their preferences and reasons. However the cables can go through fused disconnect switch for ptotection and isolation provision.

 

[marks1080]

My question is more regarding the bonding conductor from the shield to ground, not the specific application.

We happen to use extremely high capacitance CVTs in the switchyard; high frequency surge protection is a very important issue, I've got a bunch of IEDs with burnt out voltage inputs to prove it

Has anyone else had to deal with this issue? We are currently going through type testing and approvals for gland style connections for cables which will reduce the bond length practically to zero, but in the meantime there are some stations that require immediate attention.

Thanks,
Mark

 

[collies99]

We use Positron surge protection on sensitive telecom devices, wonder if it can work for your application.

 

[marks1080]

I will look into it, thanks for the heads up. I'm of the opinion that relay manfacturers should be putting surge suppression inside their relays as opposed to the user trying to mount some cumbersom circuit on terminal blocks on the back of a panel. It's also good to eliminate the problem in the switchyard whenever possible. Especially if its something as easy and cheap as proper grounding techniques...

Thanks for the suggestion!
Mark