Disconnects & Overcurrent Protection

Question 1

Properly sized and located, overcurrent protection device(s) will:
1. protect only the PV modules from high Isc.
2. protect a conductor from current exceeding the conductor’s ampacity.
3. protect grounded conductors from excessive voltage.

Answer 1

2. protect a conductor from current exceeding the conductor’s ampacity.

In any electrical system, overcurrent devices are used to protect conductors from current levels exceeding their ratings. The conductor must be protected from all power sources connected to it – for example, multiple PV source circuits in parallel

Question 2

true or false. The primary purpose of overcurrent protection (circuit breakers and fuses) is to prevent conductors from overheating and potentially causing a fire

Answer 2

true

The primary task of overcurrent protection devices is to protect conductors from carrying too much current, which would cause overheating, and potentially a fire

Question 3

true or false. The rating of the overcurrent protection device must be greater than the ampacity of the conductor it protects

Answer 3

false

In order for overcurrent protection to properly function, it must activate before the current in the circuit exceeds the rating of the conductor. The overcurrent protection should have a rating equal to or less than that of the conductor. In NEC Section 240.4(B), there is an exception to this general rule, allowing the use of the next higher standard size overcurrent devices to protect a conductor

Question 4

At a grid-direct string inverter, ___________ conductors must have a _________ means to isolate the inverter from all power sources.

Answer 4

ungrounded; disconnecting

The National Electrical Code (NEC) requires all ungrounded conductors to have a disconnecting means

Question 5

Which ONE of the following can function as both a load-break disconnecting means and overcurrent protection?
1. Blade switch
2. ATC fuse
3. Fuse in a touch-safe holder
4. Bus bar
5. Circuit breaker

Answer 5

5. Circuit breaker

Circuit breakers can be manually operated and reset, allowing them to function as both disconnects and overcurrent protection. Fuses, even in touch-safe holders, should never be opened under load - arcing and fires can result if they are opened while current is flowing through them.

Question 6

In order to isolate components from all sources of power, each piece of equipment in a PV system must have ___________________.
1. rapid shutdown devices
2. grounded conductors
3. disconnecting means
4. fuses
5. ground-fault protection

Answer 6

3. disconnecting means

Disconnects are required for emergency system shutdown, and to isolate equipment from power sources so that it can be serviced or replaced. All sources of power must be disconnected from the piece of equipment to safely service it. In PV systems for example, the inverter must have some sort of disconnecting means on both the DC and AC side

Question 7

Choose THREE specifications that fuses used to protect PV source circuit conductors must meet:
1. AC rated
2. Be supplied by the module manufacturer
3. rated for maximum system voltage
4. DC rated
3. UL listed or equivalent

Answer 7

3. rated for maximum system voltaget
4. DC rated
5. UL listed or equivalent

In addition to being properly sized, PV source circuit fuses must have a voltage rating equal to or greater than the maximum system voltage, and they must be DC rated. In addition, fuses must be listed by UL or another equivalent testing agency

Question 8

Overcurrent protection in DC PV circuits should be sized at minimum for: (hint its an equation)

Answer 8

Isc x 1.25 x 1.25

Overcurrent protection in DC PV circuits must be sized to carry the calculated maximum continuous current, defined as short circuit current (Isc) x 1.25 x 1.25. This ensures that the fuse or breaker will not trip due to greater than rated PV output at high irradiance levels, and that it doesn’t operate at more than 80% of its rating.

Question 9

When a fuse blows or a circuit breaker trips, it _____________ the circuit, protecting the conductor.
1. opens
2. energizes
3. closes
4. shorts

Answer 9

1. opens

Overcurrent protection is designed to protect conductors from current levels above their rating. In order to accomplish this, the overcurrent protection opens the circuit, stopping the flow of current