TRANSFER SWITCHES (WPTZ)
This category covers automatic and nonautomatic transfer switches,
including associated control devices, with maximum ratings of 600 V ac and
transfer equipment over 600 V ac but not more than 38 kV.
Transfer switches rated 600 V and less intended for use as service equipment
are marked ??SUITABLE FOR USE AS SERVICE EQUIPMENT.?? Transfer switches rated over 600 V intended for use as service equipment may be
marked ??SUITABLE FOR USE AS SERVICE EQUIPMENT.??
Transfer switches are required to be designed so that the load cannot
remain simultaneously disconnected from both the normal and alternative
sources when either or both sources are available, except that transfer
switches marked ??SUITABLE FOR USE AS SERVICE EQUIPMENT?? are
provided with externally accessible means to independently disconnect both
the normal and alternate sources.
Automatic transfer switches transfer a common load from a normal supply
to an alternate supply in the event of failure of the normal supply, and
automatically return the load to the normal supply when the normal supply
is reestablished.
Additional sensing devices that may initiate or delay transfer have been
investigated in accordance with the manufacturer?s marked operating values.
Automatic transfer switches may have a switching contact to initiate the
starting of an engine generator set.
Transfer switches have been investigated for load switching and inrush
capability and for a number of cycles of operation based on their intended
use which, in the case of an automatic transfer switch, is expected to
include scheduled test operations switching full load.
Listed transfer switches rated 600 V or less, without enclosures, are
intended for use as part of other equipment or where open type devices are
acceptable. These devices have the Listing Mark applied to the transfer
switch panel. When the Listing Mark is applied to the enclosure of an
enclosed transfer switch or bypass switch, it indicates the Listing of the
complete enclosed assembly.
Transfer equipment rated over 600 V is investigated only as a completely
enclosed assembly.
Transfer switches without integral overcurrent protective devices are suitable
for continuous use at 100% of rated current. Transfer switches incorporating
integral overcurrent devices are suitable for continuous use at 100%
of rated current unless restricted to use at 80% of rated current, as indicated
by the marking ??CONTINUOUS LOAD CURRENT NOT TO EXCEED 80
PERCENT OF SWITCH RATING?? on the switch.Transfer switches are rated in amps and are generally considered to be suitable for total system transfer, which includes control of motors, electricdischarge lamps, electric-heating loads, and tungsten-filament lamps within
the amp rating marked on the nameplate.
Unless marked otherwise, transfer switches rated 600 V or less, and rated
100 A or less are suitable for use on circuits having available fault currents
not greater than 5000 A rms symmetrical. Transfer switches rated 600 V or
less, and more than 100 A are suitable for use on circuits having an available
fault current of 10,000 A rms symmetrical or 20 times the transfer switch rating, whichever is greater. Transfer equipment rated over 600 V is
suitable for use on circuits having available fault currents not exceeding the
rating marked on the equipment.
Transfer switches rated 600 V or less that are marked with a short-circuit
rating without reference to an overcurrent device by manufacturer and catalog number have been tested for a minimum of three electrical cycles and
are intended for use with a Listed molded-case circuit breaker without an
adjustable short-time function.
Transfer switches rated 600 V or less, and having manual operators accessible
only by opening the enclosure are not intended to be manually operated
under load.
Some transfer switches may be provided with ground-fault protection for
services or major feeders. The circuit(s) so protected are identified by a
marking, such as on a wiring diagram.
Transfer switches without integral overcurrent protection are marked to
indicate the maximum rating of overcurrent protection to be provided ahead
of the transfer switch.
These transfer switches are intended for use with copper conductors
unless marked to indicate which terminals are suitable for use with aluminum conductors. Such marking is independent of any marking on terminal
connectors and is indicated on a wiring diagram or other readily visible
location.
Unless the device rated 600 V and less is marked otherwise, the wiring
space and terminations are based on 60?C wire for switches rated 100 A or
less, and 75?C for switches rated more than 100 A.
Unless transfer equipment rated over 600 V is marked otherwise, the wiring
space and terminations are based on the use of Type MV90 conductors.
The ampacity of Type MV90 conductors is specified in Tables 310.75 and
310.76 of ANSI/NFPA 70, ??National Electrical Code.??
If it is OK, what about nuetral bonding at the existing main disconnect?
