SMA SB 4200TL HC Multi-String - Manual (Page 52)

Rating for a Line Circuit Breaker SMA Technologie AG
Page 52 SBTLHCMS-IEN080411 Installation Guide
When selecting line circuit breakers, a number of load factors need to be taken into
account. These can be found in the respective data sheets.
For example, one manufacturer's circuit breaker may be designed for an ambient tem-
perature of 50 °C.
Load factors according to data sheet specifications:
•
Reduction through permanent load >1h = 0.9
a
• Reduction when 9 circuit breakers are arranged side-by-side without gaps = 0.77
b
• Increase in nominal current as a result of ambient temperatures of 40 °C in the
circuit breaker panel = 1.07
c
Result:
The nominal load current for the line circuit breaker is calculated as:
Ibn = 25 A x 0.9 x 0.77 x 1.07 = 18.5 A
Summary:
The selected line circuit breaker cannot be used in our example case since the maximum
current-carrying capacity for fault-free operation is lower than the maximum output cur-
rent of the inverter used.
It will trip under rated operating conditions!
In this case one solution would be to ensure there is an 8 mm gap between the circuit
breakers. This would mean that the reduction factor is 0.77 instead of 0.98. As a result,
the maximum current-carrying capacity would be 23.6 A.
In addition to the thermal rating of the circuit breakers, the boundary conditions as laid
out in section "Rating for a Line Circuit Breaker in a Photovoltaic Electrical Power Unit
Operated in Grid-parallel Mode" (Page 17) and the applicable DIN VDE standards
also need to be taken into account, of course. The main ones that apply here are:
•
DIN VDE 0100, part 410
• DIN VDE 0100, part 430
• DIN VDE 0298, part 4
In special applications the relevant standards must be followed.
Example for the thermal selection of a 25 A line circuit breaker with B sensitivity
with no gap between the circuit breakers:
a. Permanent loads longer than 1 hour are possible in photovoltaics.
b. When only one circuit breaker is used, this factor = 1
c. Due to the fact that the circuit breakers are rated for 50 °C.