3333KVA dry substation question 1

[echap]

We are doing a project here that will have (2) 4,000A 480/277V 3PH 4W dry type indoor substations. The primary voltage is 12,470/7200V 3PH 4W. We are planning an S&C PMH-12 pad mounted switch for the main service switch called "PS-1". The building owner will own this switch. Out local utility distributes in 4W (WYE type configuration).

My question is, would it be appropriate to bring 3 wires (Delta) from the main service switch (PS-1) (leaving the utility Neutral behind at PS-1) and have a Delta-Wye dry type substation transformer connection?

The transformers in the substations would be 2500AA/3333FA KVA VPI-Polyester air cooled. The substation secondary would be 4000A, 100% rated bolted pressure switch with GFI protection at 480/277V 3PH 4W.

Thanks,
Eric

 

[wareagle]

I would bring the 3 phases and the equipment grounding conductor. The primary voltage is 12,470/7200V 3PH 4W. You need a path for a ground fault.

 

[echap]

Yes, of ocurse we would bring the ground conductor. Would it be OK to use a Delta-Wye transformer?

 

[davidbeach]

No problem with a delta-wye transformer. But I would seriously reconsider that bolted pressure switch and use a breaker instead.

 

[echap]

Thanks for your input Davidbeach, but why the reservations on the BPS? It's a large cost savings.

Eric

 

[davidbeach]

It's a large cost savings.

Ever heard the expression "You get what you pay for"? You are absolutely correct, the BPS is far less expensive (read cheaper). That's about all that can be said for it though. So you blow one fuse, then what? How do you clear the other two phases to protect the load against single-phasing? Can you be assured that the cheap customer will replace all three fuses with a new, matched, set of fuses each time one fuse blows? As far as I'm concerned, the BPS is something for a contractor to install when provided with no other direction than CHEAP, but any Engineer should be looking at the big picture and provide a better long term value for the client. Nothing I've seen suggests that a BPS provides long term value.

 

[PWR]

Being in the service business, I have seen dozens of bolted pressure switch failures. I have also seen many used successfully over an extended period. The big issue is maintenance. Most facilities that go the cheap route (BPS versus breaker) don't do any maintenance on them. Many breakers also don't get serviced but they generally can withstand such abuse better than a BPS.

 

[rbulsara]

I agree with davidbeach and PWR. Recently I got involved in a project because a 4000A BPS failed to open on a ground fault because of a mechanical issue (the relay worked but switch mechanism got stuck). That led to severe arcing and burndown of the board, forcing a big hotel to shutdown. Good breakers are much less likely to fail like that.

 

[tulum]

Hey folks.

(1) Not quite familer with the term "bolted pressure" anyone care to expand?

(2)David beach; "Can you be assured that the cheap customer will replace all three fuses with a new, matched, set of fuses each time one fuse blows"

I here what you are saying but could the same not be said for breakers? i.e. is there not a finite quantity of fault operations that a breaker can handle before needing to be repalaced or tested. When a breaker trips the electricians I deal with find the problem fix it and then the old breaker goes right back into service...

Regards,
TULUM

 

[davidbeach]

While a breaker is good for a finite number of fault interruptions, that number is far greater than 1. Applied within its rating, a breaker is likely to fail of neglect and old age before it fails as a result of doing its job. Better for the breaker to have one operation a year than for it to never be operated.

I'm not clear enough on the exact internal construction of the BPS to describe it, but it is basically a high amperage fused disconnect that can be tripped by a ground fault unit.

 

[rbulsara]

Bolted pressure or High Pressure Swichtes are the terms used by switch manufactures in the USA that refers to the switches that incorporate spring loaded (pressure) mechanism that yields hih speed contact movement independent of the speed of movement of the operatoring handle (or operators hand.

Bolted I think refers to the fact that it is not drawout type, but not sure.

These are some of those popular and normally undestood (in the USA)terms but may or may not make technical sense. (not saying this one does not)

 

[tulum]

Thanks guys...

I used a boted pressure switch in a Toshiba JK power cell before... but never knew exactly what the term meant (still a little unsure). The switch was a non-load break (good for 0.4 at 5kV - scary) and was not spring loaded... it was a mechanical marvel that was as slow as schnitzel...

Regards,
TULUM

 

[tinfoil]

@tulum:

When one phase of three phase fuse installations blow, one or more of the others may have been exposed to near-melt conditions; not enough to go, but enough to degrade its current capacity. The next time the service goes to peak, or perhaps during the next enerergization inrush event, this fuse pops. The troubleshooters usually then spend time looking for a defect in the system, when the problem was in the protective fuses.

 

[waross]

Bolted pressure;
A bolted pressure is a type of knife type disconnect switch.
It may or may not be fast acting.
The mechanism closes the switch blades and then continues farther to turn special threaded bolts to clamp both ends of the switch blade, ensuring a good connection. When the switch is opened, the operating linkage first turns the special bolts to relieve the pressure on the connection and then opens the switch.
The feature described by rbulsara may be incorporated in a bolted pressure switch, but is not the bolted pressure feature.
A non-spring loaded switch as described by TULUM may have the bolted pressure feature.
My experience has been with some that are used unfused (but fusible) for disconnecting 480 volt transformer banks. Mainly for maintainace isolation, but able to open under load. These were manual switches. Very old SquareD. 1600A or 2000A rated current.
respectfully

 

[rcw retired EE]

Bolted Pressure switches require maintenance. So do 4000A circuit breakers but they seem to be a little more forgiving. I have had to use hydraulic jacks (Enerpacs) to pull bolted pressure switch contacts apart during maintenance. The contacts would not move when we hit the trip button or tested the ground fault. (Contact utility, kill power to the hospital and do maintenance.) Other switches melted and burned down when they hung up during a ground fault trip, leaving only a portion of the switch surface making contact.

A bolted pressure siwtch will save intial cost and can be a good choice in a well-maintained facility. Sometimes the increased short circuit capability of the fused BPS makes it a better choice over circuit breaker switchger with current limiting fuses.

 

[waross]

The bolted pressure switches I have worked with were used as manual disconnects. There was no provision for either automatic opening or closing. In this application I considered them to be a much better switch than a plain disconnect switch.
However, I don't think I would be impressed by any attempt to automate one. As a manual switch it has excellent current carrying capacity and long life. As an automatic switch, (other than a remote operated isolating switch) I don't think it could match the capabilities of a good breaker. I've opened and closed bolted pressure switches many times under load, but I wouldn't want to open one under a fault.
respectfully