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Topic Title: Programmers
Topic Summary: Definition NO and NC
Created On: 20 February 2014 05:24 PM
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 20 February 2014 05:24 PM
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nige296

Posts: 76
Joined: 04 December 2009

Sorry for the 'bone question' but I understand that on the honeywell st699B terminal 6 is HW on, so why does it show NO (contacts normally open), surely with the contacts open there is no current and therefore the HW is off! confused!!
 20 February 2014 05:28 PM
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typiod

Posts: 200
Joined: 09 February 2008

Thats correct when the time switch then calls for hot water the contacts then become closed.
 20 February 2014 05:37 PM
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nige296

Posts: 76
Joined: 04 December 2009

"Thats correct when the time switch then calls for hot water the contacts then become closed"in that case the switch goes to 7 (NC) which is HW off!! still confused?
 20 February 2014 05:39 PM
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typiod

Posts: 200
Joined: 09 February 2008

The hot water off is only needed for a y plan so that the valve will move
 20 February 2014 05:41 PM
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intrinsic4225B

Posts: 1626
Joined: 30 September 2004

The terms normally open (NO) and normally closed (NC) usually refer to state of the contacts when a system or device is not energised.

If we consider, for example, a relay that we have just taken off the shelf and which is not connected to anything, the NO contacts will be held open by mechanical force and will close when the relay coil is energised.

Conversely, NC contacts will be held closed by mechanical force and will open when the relay coil is energised.
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