Key
interlocks are available in a variety of configurations including transfer
blocks to capture keys from multiple sources. They are often used as part of
Lockout and Tag-Out procedures. It is recommended that the design engineer
refer to a key interlock manufacturer such as Kirk or Superior for further
documentation and specific operational details.
Drawing Notes are extremely important
as they describe specific functional requirements. In Figure 1.1-12,
Note 3 above switchgear “SUS-F1A” describes an additional requirement for a
Priority Load Shed Scheme to ensure the generator is not overloaded. The
details of this scheme would need to be coordinated with the generator
manufacturer and further defined in the switchgear specifications.
Note 4 calls
for DT1150+ electronic breaker trip units that include an Arcflash Reduction
Maintenance Mode. This feature limits arc flash energy in compliance with
Article 240.87 of the 2014 NEC by using an alternate high-speed analog
instantaneous trip setting to reduce arcing time. Note 5 requires a touchscreen
panel to monitor the operating variables as well as be used to activate the Arcflash
Reduction Maintenance Mode remotely.
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| Figure 1.1-11. Drawing Notes and Key Interlock Scheme in LV Switchgear |
This
permits personnel who will be working on the equipment to be in a safe location
outside of the arc flash zone when enabling the Arcflash Reduction Maintenance
Mode. Note 4 also requires Zone Selective Interlocking. This feature permits
higher speed tripping of the Main breaker, if it does not receive a restraining
signal from a downstream feeder breaker that it is tripping to clear a fault. Note
6 adds a requirement for BACnet communications functionality to a future Building
Management System. It also provides a point of demarcation between the scope of
work to be provided by the installing contractor and what portion of the wiring
and interface will be required of the BMS vendor. Each of the circuit breaker
symbols in the
“SUS-F1A”
switchgear are surrounded by double arrows signifying that these breakers are drawout
versus fixed mount. Additionally, the “E.O.” nomenclature in the middle of the
breaker symbol represents “Electrically Operated”. This function makes it
easier to open and close the breaker. It also enables the opportunity for
remote control from a handheld pendant operating station or a wall-mounted
control panel.
Each
circuit breaker is named and its Frame Size (AF), Trip Rating (AT) and
protective functions such as Long, Short and Ground (LSG) or Long, Short,
Instantaneous and Ground (LSIG) are noted accordingly. Since this equipment is
drawout UL 1558 switchgear, the 4 cell high structure number and associated breaker
cell are illustrated.
“Spare” breakers have been located in the top “A” cells
02A and 04A as well as cells 04B and 4C in structure #4. The generator breaker
is also located in top cell 03A of structure #3, for cable and conduit egress
out the top. In this example, all other breaker cables “feeding loads exit out
the bottom of the switchgear. This avoids bottom exiting cables from covering access
to the lugs for the spare and generator breakers. Consequently, it permits room
to terminate the future cables, coming into the top of the switchgear, easily
at a later date.
It is always wise to include spare breakers of important frame
and trip sizes in a drawout switchgear lineup. These spare breakers can either allow
for future load growth or provide a readily available backup that can be used
in the event that an active breaker requires maintenance or service.
Note that
interference interlocks are supplied on breakers and in switchgear compartments
where the compartments are of the same physical size. This rejection feature
ensures that an insufficient short circuit or incorrect ampacity rated breaker cannot be inserted into the wrong size cell.
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| Figure 1.1-12. Drawing Notes and Key Interlock Scheme in LV Switchgear |
As an example, a
1600 A breaker cannot be used in a cell configured for 800 A as it would not
likely protect the cell bus runbacks and outgoing cables appropriately.
Likewise, a 65 kA short circuit rated breaker could not be inserted into a
switchgear cell rated for 85 kA.
Figure 1.1-12 shows the main bus for
the switchgear rated at 4000 A with an 85 kA short-circuit rating. A busway
symbol is illustrated above the tie breaker, indicating that it is connecting
to the other half of a double-ended switchgear lineup.
Eaton’s low-voltage
busway can be supplied in ratings of 6–30 cycles. The 4000
A busway shown has a 200 kA 6 cycle rms symmetrical short-circuit rating that exceeds
the 85 kA rating of the “SUS-F1A” switchgear bus on the drawing.
The calculated
short-circuit rating required for the “SUS-F1A” switchgear is dependent on a
number of factors including: the available short circuit upstream, the
inclusion of the cable and transformer impedances feeding it, as well as the
short-circuit contribution from the motors downstream.
In actuality, the short circuit
current available may be lower than the 85 kA shown on the drawing, permitting
a potential cost and space savings, if the rating required is dropped to 65 kA
or below. A short circuit study would need to be done to confirm this.
Consequently,
it is very important to indicate the actual breaker short-circuit rating as
well as the switchgear bus ratings on the One-Line. These also need to be consistent
with other schedules and drawings, as well as in the equipment specifications.
This can prevent a bidder from incorrectly quoting 85 kA rated switchgear with
65 kA rated breakers.
The System One-Line shows the incoming surge protective
device (SPD) in “SUS-F1A” is rated at 250 kA per phase. As shown in Figure
1.1-13, SPDs in the other downstream equipment are rated at 120 kA per phase.
This surge protection scheme as shown is applied in a tiered approach per the
IEEE Emerald Book. In this arrangement, the highest level of surge protection
is at the incoming source. Downstream switchboards or panelboards closer to the
loads provide the next of surge protection.
There is a considerable amount of distribution
equipment illustrated on the example System One-Line. For that reason,
reference is made to other drawings and schedules that would comprise the
hypothetical bid package. As an example, 1600 A distribution switchboard DSB DF4A
has a note to see schedule DSB-DF4A for the end loads. The same is true for
power panel PP-DF6A.
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| Figure 1.1-13. Distribution Equipment Downstream of the SUS-P1A Switchgear |
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