Non Class 480 Load Centers& MCC (NG/NH)

Question 1

What’re load centers for ?

Answer 1

Transforms 13.8kv(NA) power to 480VAC powering:
Non class loads >50HP

480VAC MCC <50HP

Question 2

Describe flow path of 480VAC(NG)(NH)

Answer 2

From13.8(NA)
Transformed to 480vac
Distributed to >50HP load
Also distributed to MCC(NH) for <50HP loads
MCC steps down 480VAC->120VAC for distribution panel

Question 3

What is the disconnect switch inside load center cubicle for ?

Answer 3

To be able to isolate power from NA system, without losing power to the other load centers

Question 4

What components inside the load center interface with Kirk key interlock ?

Answer 4

The disconnect switch & the B2 main feeder breaker

Question 5

What is the Kirk Key interlock for ?

Answer 5

Safety feature to prevent disconnect switch from being opened under load

Question 6

How does Kirk Key Interlock work ?

Answer 6

Requires B2 break to be TRIPPED open to allow you access to a key. The key allows you to operate the disconnect switch

Question 7

Major operating steps to operate Kirk key interlock ?

Answer 7

.Open B2 Breaker
.press breakers trip button and pull o it locking hasp
.press plunger below key lock
.turn key and remove it
.insert key below disconnect and turn
.rotate disconnect switch handle down 180 degrees

Question 8

What voltages does load center transformer to?

Answer 8

13.8KV —>480KV

Question 9

What’re the load center transformers cooled with ?

Answer 9

Natural circulation of air from vents on the cubicle

Question 10

What’re the load center transformers located by cooling towers cooled by ?

Answer 10

Cooled from natural circ. Through cooling fins

Question 11

What happens on a over current trip on ?

Answer 11

It will trip 13.8kv supply breakers on NAN-S01/S02 and the 480Main Feeder breaker

Question 12

What’re circuit breakers meant for ?

Answer 12

To break circuit on a fault isolating equipment from power source

Question 13

What’re Motor Feeder Breakers for ?

Answer 13

They’re equipped with longtime/instantaneous overcurrent for tripping & must be reset locally after trip

Question 14

What is the control power for loadcenters ?

Answer 14

125DC (NK)

Question 15

When are the closing springs charged on 480Load Center ?

Answer 15

Closing springs recharge upon OPENING of breaker

Question 16

What does RED light mean on breaker ?

Answer 16

Breakers closed and trip circuit has continuity

Question 17

What does GREEN light mean on breaker ?

Answer 17

Breaker is open (If light shines brighter there is a fault)

Question 18

What does the blue light mean on the Cooling tower Load Centers breakers ?

Answer 18

Will trip due to high vibration of the fan

Question 19

What control power do MCC’s use ?

Answer 19

It uses its own power for control power; it sends its 480VAC power through transformer that steps it down to 120VAC for control power

Question 20

What happens to MCC control power if 480VAC to the MCC is lost ?

Answer 20

The MCC will lose control power because it will be unable to supply itself

Question 21

How does NH system interface with class 480VAC load centers (PG) system

Answer 21

PG system supplies power to non class SIAS load shed MCCs:
NHN-M19
NHN-M20
NHN-M71
NHN-M72

Question 22

What indication will Control Room get on a Load Center fault?

Answer 22

Control room will receive a trouble alarm