Relay And Contactor Terminology

http://en.wikipedia.org/wiki/Switch

First see the Wikipedia site.

Now, 4-pole means four switches with one actuator (handle).

Single throw means that a switch is either ON or OFF. There are two connection points. 4PST (4 pole, single throw) is four of these put together with one actuator.

Double throw means that the switch can be ON in either position, but will be OFF in either position also. There are three connection points. 4PDT (you get the picture) is four of these put together with one actuator. This type may have one circuit ON and three circuits OFF in one actuator position while the other actuator position will turn three ON and one OFF. Likewise for two circuits ON, two OFF and vice-versa.

bill,

So If I am running (say) 2 sets of 4 lamps each, alternatively.

So I can use a 4 pole, double throw relay right?

I am not sure what you mean with the 2 sets of 4 lamps alternatively, please give little sketch or something.

With regards to the double throw question, I don't think the answer was very clear.

Single throw means you have one switch wich is either on or off.

Double throw means you have 2 switches with a common. This means you always have one switch off and one switch on.

If you now have a 4 pole relay it just means you have 4 of the above together on one actuator or relay.

single

double

Yes.

There are lot of Terms related to Contactors & Relays:

Contacts: [For all disconnectors like Contactors, Relays, Safety-cutouts
[Over Temperature etc etc ]Switches & like ]

NO = Normally Open. Will make on energisation of Contactor or Relay
NC = Normally closed [made]. Will open on energisation.
SP = Single Pole
DP = Double Pole
ST = Single Throw
DT = Double Throw
CO = Centre OFF
3T, 3P [Digit+ P for multi-poles digit indicating the number of poles ]

Aux contacts:
[CBs & Contactors only] Low-current contacts on side [ or any-where ] used for Auxiliary functions like:

NO = are used normally to hold the contactor [by-passes the "Start" button etc ], also to energise other circuits/ devices.

NC = are used normally to disable devices/ circuits.

These are normally marked A1, A2, B1, B2 etc or may differ somewhere in the world.
In CBs also used as End-Stop of its motor in Make or Break Positions.

Main Contacts:
[CBs & Contactors only] Hi-current contacts:

Most of the contactors are normally open & Make on energisaion.

These are normally Marked 1, 3, 5 on one side & 2, 4, 6 on opposite side.
i.e. 1 & 2, 3 & 4 and 5 & 6 make contacts.
Coils are in all relays, CBs & Contactor, but in CBs there are Aux Coils i.e. :
Over-Current Trip Coil: Disconnects CB if Overloaded
* Shunt-Trip Coil : If voltage comes on this coil CB will break.
Normally monitors other Supply source & will trip CB if present

** No Voltage Coil: Will not allow CB to make if Voltage not present

* Functions mechanically to actuate

** Functions mechanically to block
and more ...

To add:

Hi-Power CBs [Circuit breaker] are motor driven to make or break ....