Vault disconnect switch on Primary Subs

[nsbelectrical]

I have 34.5KV line to 4160V. On the primary side of the transformer there is a vault with a disconnect switch (3 phase). This is fed from an overcurrent protection device some distance away. There has been problems with the disconnect switch so Im looking into getting rid of it, and I was wondering if it is "legal"? Is there NEC code requiring the switch, and/or any other issues that might arise from getting rid of the switch.

Be Safe.

 

[dpc]

In general, the NEC probably does not require a local disconnect at a transformer, but at 34.5 kV, I'd want to be sure that there was a reasonable way to isolate the transformer and a place close to the transformer where protective grounds could be applied to the transformer primary for worker safety.

But if this is owned by a utility, the NEC does not apply.

 

[nsbelectrical]

Thanks for the input dpc:

This XFMR is owned by our facility. Our overcurrent device is ~ 800 ft away and we can apply grounds there.

 

[Zogzog]

What problems are you having with the switch?

 

[nsbelectrical]

We have had some mechanical issues with the switch opening/closing, but more recently and more importantly we had a switch burn up. I'm not sure of the details, but I will know tomorrow and post them here.

 

[dpc]

If you have any overhead line between the overcurrent device and the transformers, you really need to apply safety grounds close to the transformer.

 

[nsbelectrical]

These lines are in conduit (underground)

 

[nsbelectrical]

To Zogzog:

The insulation around the switch had moisture build up on it. This created a low resistance to ground resulting in some arcing and ultimately causing damage to the switch, insulation and vault. NOTE: we do have heaters inside the vault to avoid this problem but it seems they had some type of failure.

 

[cuky2000]

Most switches at 34 kV are not rated to interrupt the load or event a transformer without load.

What kind of switch do you have in this facility?.
Did you check with the switch manufacturer for this application?

 

[nsbelectrical]

This is just a disconnect switch on the primary side of a 34.5KV to 4160V transformer. The switch is located inside a vault. It is not intended to be operated under load.

 

[Zogzog]

Are (were) the heaters working? You can replace the insulators in that switch probally for less $ than removing it and splicing.

 

[nsbelectrical]

The heaters were not working. These heaters are notorious for not working and this has to do with the fact that they cannot be serviced while the XFMR is under load. So we are looking into removing the disconnect and desiging a vault with a heating mechanism that can be extracted from the vault (while under load) and serviced.

 

[dewone]

Have you done any service work on the switches. ie lube the mech, clean and wax the insulators, check the switch operation. As earlier stated it would be cheaper, to repair and do proper maintenance vs replacement. I agree you need to modify the heater situation.

 

[nsbelectrical]

Dewone:

I agree with you and zogzog that it will be cheaper to service the switches rather than removing them. We our using a silicon to lube the mech., we do check switch operation periodically, and Im not sure how often the insulators have been cleaned. We will be changing to a more reliable heater and possibly putting a screw plug type heater into the side of the enclosure with a metal casing surrounding it thus allowing us to replace the heater element without de-energizing. I was interested in removing the disconnect switch because we will purchasing new Primary subs and will likely get rid of the disconnect switch being that it is "legal" to do so. However we still need to heat the vault to ensure no moisture build up on the primary side of the XFMR.

thanks.

 

[Zogzog]

If you are getting new primary subs, mount the close/trip switches near the entrance to the sub away from the gear. This option dosent cost much more to do and can eliminate the arc flash hazard for switching. Few people do this and it one of the best safety upgrades you can do. Also look into putting IR windows in for IR scans, all OEMS have these now.

 

[nsbelectrical]

Im not sure exaclty what you mean with the close/trip switches ... currently we have a close/trip switch followed by a SF6 CB about 800 ft away from the PSub - this runs gto a HV disconnect inside a vault on the primary side of the sub, then we have the XFMR and switchgear.

Thanks for the info. on the IR scans I will look into putting those on the vault doors of the new subs for preventive maintenance.

 

[Zogzog]

I was refering to your 5kV gear. I reread your post, I guess you are only replacing the 34.5kV equipment.

 

[nsbelectrical]

Thats correct. We havent had any problems with the 5kV gear. On my previous post I was refering to IR windows on the vault doors... not IR scans

Thanks and

Be Safe.