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When the load is controlled from only two points, single pole, double throw (SPDT) switches are used. Double pole, double throw (DPDT) switches allow control from three or more locations. In alternative designs, low-voltage relay or electronic controls can be used to switch electrical loads, sometimes without the extra power wires.
The second transformer is connected to a center-tap of the first transformer, and is wound for 86.6% of the phase-to-phase voltage on the three-phase system. The secondaries of the transformers will have two phases 90 degrees apart in time, and a balanced two-phase load will be evenly balanced over the three supply phases.
For example, a "2-pole" switch has two separate, parallel sets of contacts that open and close in unison via the same mechanism. The number of "throws" is the number of separate wiring path choices other than "open" that the switch can adopt for each pole. A single-throw switch has one pair of contacts that can either be closed or open.
A double-switched relay cannot close inadvertently with the application of the same current. At least two separate faults would be required to allow this relay to close inadvertently. Double switching , double cutting , or double breaking is the practice of using a multipole switch to close or open both the positive and negative sides of a DC ...
Each row of breakers in the distribution board is fed from a different phase (L1, L2, and L3), to allow 3-pole common-trip breakers to have one pole on each phase. The distributor's service head will contain a main fuse for each phase. Nominal voltage is usually 230 V AC between phase and neutral, and 400 V AC between phases.
SPST-NC (Single-Pole Single-Throw, Normally-Closed) relays have a single Form B or break contact. As with an SPST-NO relay, such a relay has four terminals in total. SPDT (Single-Pole Double-Throw) relays have a single set of Form C, break before make or transfer contacts. That is, a common terminal connects to either of two others, never ...