Passenger Boarding Bridges

[twagner1]

Every design I've seen at this airport supplies (PBB's) with a 480 volt, 3-phase 4-wire w/ground service of varying sizes depending on the options provided. The problem is a secondary transformer within the service disconnect that supplies 208/120 volts for lighting, heat, limits, and drive movement. This transformer could be a 71/2 kva, 10 kva, or a 15 kva in size. There is always the appropriate sub-breakers for the transformer, lighting, heating, limits, and drive circuits but think, the secondary of the transformers derives it's neutral from the equipment grounding conductor attached to the frame of the Boarding Bridge. Doesn't that make the (EGC) a current-carrying conductor? The neutral block is bonded to the can. The service neutral is dead-ended on a isolation block and never used. As a state certified electrical inspector with 11 years experience, it never ceases to amaze me of the bull-headedness of some electrical engineers. For years now I've required every (PBB) to have a ground rod poured within the footer, cadwelded and lugged to the (PBB) pedestal running solid to the secondary transformer's neutral (XO) isolated from the primary (EGC), to ensure the lost of the primary (EGC) will not injure or kill anyone. Is that overboard? Another related problem is the transformer's temperature rating (105/150 degrees) in an unventilated disconnect cooks the insulation off the wires. I've measured 210 degrees internally to the transformer! Yet, it carrys an UL listing. Simply amazing...maybe I'm wrong.

 

[LarryFine]

Re: Passenger Boarding Bridges

Originally posted by twagner1:
...the secondary of the transformers derives it's neutral from the equipment grounding conductor attached to the frame of the Boarding Bridge. Doesn't that make the (EGC) a current-carrying conductor?

No, the secondary of the transformer IS the source of the neutral, and normal neutral currents will return only to the transformer's neutral point, but not up the primary EGC.

The neutral is grounded via its bonding to the EGC of the primary. The only secondary current that should ever flow through the EGC, theoretically, at least, is if there is a secondary line-to-ground fault.

The EGC will be part of the pathway which enables the short-circuit/overcurrent device to operate; thus, the bonding of the neutral. A local grounding electrode will certainly help minimize equipment-to-earth voltages.

[ July 25, 2005, 12:28 PM: Message edited by: LarryFine ]

 

[benaround]

Re: Passenger Boarding Bridges

You can't use the EGC from the primary side on a SDS to bond XO,unless the SDS originates in listed equipment suitable for use as service equipment. 250.30 A(4)

frank

 

[physis]

Re: Passenger Boarding Bridges

the secondary of the transformers derives it's neutral from the equipment grounding conductor attached to the frame of the Boarding Bridge. Doesn't that make the (EGC) a current-carrying conductor? The neutral block is bonded to the can.

The secondary of the transformer has a neutral of it's own and doesn't derive anything from the grounding system.

If you're saying that the equipment is getting a neutral connection through the EGC then I think it's a hazard.

The service neutral is dead-ended on a isolation block and never used.

For years now I've required every (PBB) to have a ground rod poured within the footer, cadwelded and lugged to the (PBB) pedestal running solid to the secondary transformer's neutral (XO) isolated from the primary (EGC), to ensure the lost of the primary (EGC) will not injure or kill anyone.

Is there a reqirement for that?