Just when you think you’ve seen it all. Yesterday I responded to an emergency call for a tripped 4000 amp service bolted pressure switch. By the time I arrived at the job the on site engineers had reset the switch (not my recommendation) this is a service disconnect 208/120 4000 amp with blown fuse protection and ground fault protection. The engineer stated that the ground fault indictor was flagged when he first looked at the switch.
There are two services in this building the one that tripped and a 4000 amp 480/277 services. It is unusually for PEPCO (the local utility) to provide 208/120 and 480/277.
As I looked around I discovered that on the 208/120 service there is a 4000 amp 480/277 utility termination cabinet with a utility CT cabinet and a 4000 amp tap box. Connected to the tap box is the primary of a 1250 kva dry type transformer (customer owned). The secondary of this transformer feeds a 4000 250 VAC tap box which feeds the line side of the bolted pressure switch that is equipped with 4000 amp fuses. There are two inspection stickers TPF (temporary pending final) and final sticker. No line side over current protection for the transformer.
Should a customer request an electrical outage in our area PEPCO will remove the CT’s.
They lift the CT’s from an energized CT cabinet. Obviously; not in this case, and we will inform PEPCO and recommend signage to note this issue.
Anyway, we looked for the normal causes for Ground fault trips and could find none, the ground fault relay was set at 700 amps .15 seconds. The switch had not tripped again so, I explained to the customer their options, they wanted to wait for a better time and I left the site. This is a hotel and shutting down at that time was not an option they liked. Big convention in town ECT…………….
Today we got a second call from this customer; a tech went to the site and told me the following.
The main line switched was tripped, (the customer had tried to reset it and it tripped a second time, wisely they stopped trying to reset the switch). Our tech looked for obviously problems and found, that a 400 amp fused safety switch that fed 400 amp MDP
with 4 500 kcmils cu had the following A and B phase in one 2” conduit (EMT) and C and the grounded conductor/neutral in a separate 2” conduit (EMT). The conduits were hot (temperature) and the A and B phase conductors were in some places (were visible) melted into one mass of insulation and A and B phase conductors were shorted.