Give the best answer:
1. How deep do you need to make the trenches for 4800V series lighting circuits around the airport runways at a major city airport with heavily restricted public access? Assume you are using direct-burial cables.
a. 6 in.
b. 12 in.
c. 18 in.
d. 24 in.
e. 30 in.
2. If you run a 23kV circuit in direct-burial cable up the side of a building protected with conduit, how deep must the conduit extend into the earth? The conduit will be Schedule 40 PVC.
a. 6 in.
b. 12 in.
c. 18 in.
d. 24 in.
e. 30 in.
f. 36 in.
3. If you run three single-conductor Type MV cables in the earth for a 13.8Y/8kV industrial system installed under engineering supervision, which of the following conditions applies to splices in this run?
a. Must be watertight
b. Must be in boxes
c. Maintained shield continuity
d. Two of the above
e. All of the above
4. To combat condensation on an outdoor 13.8kV-power switchgear, the engineering team wants to install some 240V electric heat in the bottom of the equipment. What precautions must they take? Assume they are concerned about public access.
a. Lock equipment access door.
b. Access door interlocked with switch in the isolating position.
c. Put the heater outside and duct the heat into the compartment.
d. Bond heater to the enclosure.
5. For the equipment in the fourth question, if the access door is 38 in. by 50 in., how does it have to be configured?
a. With hinges and a door stop
b. With lifting handles
c. Both of the above
d. Either of the above
e. Not specified
6. Assume a 35kV system supplies equipment through branches arranged periodically along a tunnel about one mile long. A small-gauge railroad runs the length of the tunnel. How many bonding connections must you make between the cable shielding and the track steel over the length of the tunnel?
a. None
b. One
c. Two
d. Five
d. Six
Answers and Discussion
1. c, Table 710-4(b) Ex. 5. Although the burial depth for this voltage on direct-burial cabling is normally 30 in., in this type of restricted environment the cover depth becomes only 18 in. If even that depth is difficult, the body of the table allows heavy-wall metal conduits with only 6 in. of cover. This contrasts with Table 300-5, which requires 18 in. of cover for the same conduits running at or below 600V. The reason? Two different panels have jurisdiction.
2. f, Sec. 710-4(b)(1). Instead of merely requiring protection down to grade, this section now correlates with Sec. 300-5(d) for 600V and below. The raceway must extend at least as far down as minimum burial depth for the cable. For a 23kV direct burial cable, that means no less than 36 in.
3. a, Sec. 710-4(b)(2) and Ex. As in the case for 600V and below [Sec. 300-5(e)], underground splices don’t have to be made in boxes. They have to be watertight, and normally the shields need to be made continuous. However, under the conditions in the question, an engineered cabling system can take advantage of the buried splice point to deliberately introduce a discontinuity in the shielding, typically to prevent circulating currents in the shielding from degrading the cable ampacity. The shields do have to overlap, and they need to be grounded at a single point at both ends of the cable.
4. a, Sec. 710-24(f)(2) Ex. 2. Although usually access to such equipment needs to be interlocked so the switch is in its isolating position, the exceptions to this rule allow for space heater and instrumentation connections to be accessible under the conditions that normally apply to the equipment generally. The only restriction is covered in Sec. 710-24(f)(1), requiring a locked enclosure under these conditions.
5. d, Sec. 710-24(i). Since the door is over 12 sq ft, special restrictions apply to be sure the door doesn’t inadvertently get away from those trying to maneuver it. It can have lifting handles, or it needs to be hinged and locked or bolted. A hinged cover must have a door stop so it doesn’t shut, especially outdoors, where wind is an issue. Although product standards haven’t been requiring the stops on indoor equipment, this rule applies in all locations. The panel just rejected an attempt to confine it to outdoor applications.
6. d, Sec. 710-54(a). This rule requires bonding at the portal and intervals not over 1000 ft. On a mile run, that comes out no fewer than six times. The bonding also applies to other metal piping systems, if present.
