[brian john]

Ask him with a 12 wire lead generator what are the connections for 480/277, 208/120 and single phase 240/120?

480/277 - A Ø is 1, B Ø is 2, C Ø is 3, 4 to 7, 5 to 8, 6 to 9, Neutral is 10, 11, 12.

208/120 - A Ø is 1 to 7, B Ø is 2 to 8, C Ø is 3 to9, Neutral is 4, 5, 6, 10, 11, 12

240/120 – L1 is 10 to 8, L2 is 1 to 6, Neutral is 2,4,7,12 then connect 3 to 5 and connect 9 to 11

As a note to this, after reconnecting you must adjust the voltage to match your needs (minimally adjustments are possible after reconnecting)


But there is no free lunch here, you better study this till you know it by heart. Because when he asked you to draw it on the board you better have your "A" game on.

The easy way to remember this is draw the HV 480/277 Y with extra legs (splits between 4-7, 5-8, 6-9) start with 1 going clockwise number the ends of the Y next go round the bottom of the 1st leg second go round top of Y third go round the center of the Wye this is 480, parallel the legs for 208/120 and delta dealt for 240/120.

 

[brian john]

In a 480/277 VAC switchboard rated 1000 amps and above with a single main what is the most common GFP (Ground Fault Protection) in use today.

Residual, ground return or Zero Sequence?

In modern switchboards the Zero Sequence system is the most prevalent system used. With the advent of inexpensive large current sensor's (commonly called Window CTs) it is economical over 4 CTs of a residual system and avoids some of the problems of the ground return system.


BUT once again you better be careful stumping an instructor (unless you do some follow up reading) can get you into a bucket of well MUD.

 

[brian john]

How many amps can a 20 amp circuit breaker carry before tripping open and for how long can it carry this current?


Molded case circuit breakers are inverse time current devices, the higher the current the faster they trip.

20 amp CB at 150% (25 amps) of rating depending on ambient, a durn long time.
20 amp CB at 200% of rating (40 amps) depending on ambient 10 minutes or longer.
20 amp CB at 300% of rating (60 amps) depending on ambient 35-60 seconds.
20 amp CB at 400% of rating (80 amps) depending on ambient 10-15 seconds.
20 amp CB at 1000% of rating (200 amps) instantaneous (7-12 milliseconds including opening time of CB).

Actual operating times vary from manufacture to manufacture and the above times are estimates based upon experience.