NC contacts have very good uses. Without NC contacts, a lockout relay couldn't block close, it could only trip. Without NC contacts, how do you get the green light when a breaker is open? The statement is just a bunch of crock. Sure, an NO contact is a better choice for many things, but there are things that just can't be done without NC contacts.
I’ll see your silver lining and raise you two black clouds. - Protection Operations
The statement made in the original post is a generalization. Generalizations are often easy to disprove. I am an engineer, and I don't hate NC contacts, so the initial statement that engineers hate NC contacts is proven false by one counterexample.
There are many applications for NC contacts in addition to those davidbeach gave.
NC contacts are the preferred contacts for many stop and emergency stop functions, since opening the contacts removes power from equipment. This is a fail-safe function that allows the same stop operation as if an open occurred in a control circuit.
NC contacts are the preferred contacts for many start functions, too. It isn't easy to get an action to start with a NO process switch using hard-wired logic; it's much easier if a NC switch is used. For example, NC low level switches are often used to start a pump or open a valve to fill a vessel because they can be open during vessel drawdown, i.e., the level switch has been activated by the process liquid, then close when the level drops below the set point. Same goes for turning on conveyors, fans, etc., - a NC switch may be needed for such applications.
Many other examples exist...
I do admit that when NC switches fail to open due to operating mechanism issues or welded contacts they can cause significant problems when used in safety applications. That's why safety relays or other monitored circuits should be used in those designs.
xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
I know many people don't like the term "normally" and this can create confusion - sometimes leading to hate.
"Normally" is the state of the contact when the coil is de-energised (or reset if latching).
However there are many applications where the coil being energised (or set) is normal - for example supply supervision.
In the supply supervision example, if you want a contact to raise an alarm if supply is lost to a supervision relay, you need to use a Normally Closed contact.
However the NC contact will be normally open for this application and only closed when things are not normal
When I first started here, we used the manufacturer's standard NC contact held open by the relay as an indication of relay failure. If the relay failed, the contact would close. Closed, aka continuity through the circuit, meant Relay Failure. Ten or more years ago we stood that on it's head and have since used a NO contact that the relay holds closed. Closed now means Relay Healthy; Relay Failure is the absence of Relay Healthy. For the relay itself the change really doesn't mean much. But what it does mean is that an open connection or broken wire any place in that circuit removes the Relay Healthy indication, signifying Relay Failure, but in the prior way of doing things an open connection meant that we'd never get Relay Failure.
I’ll see your silver lining and raise you two black clouds. - Protection Operations
Safety relay applications require the "external device monitoring" circuit of the safety relay be routed through the NC contacts of the power contactors that the safety relay is operating, controlling, and monitoring. The contactors must be the forced-guided type in which it is mechanically impossible for the NC and NO contacts to be simultaneously closed. If a relay sticks in the energised position, this holds open the EDM circuit of the safety relay and prevents reset.
So, no, I don't hate them ... they perform a necessary function.
Hi David - yeah we do the same where the relay allows.
We use a Normally Open supervision contact that will be closed when everything is OK, and then invert the singal in SCADA so that the alarm is raised when the SCADA input is low.
It is a much better supervision system, however it creates even more confusion for "non-relay" people.
Was just trying to make the point in that "Normally" creates the confusion when the use of the relay is not "normal" - with a simple example.
The basis of the statement might be because historically some relays had N/C contacts that were not rated for the same duty as N/O contacts. Hence, for certain uses the N/O contacts had to be used. Some still are that way, but it still doesn't preclude using the contacts when they are needed.
Yeah DiscoP, every time I do training on relays for non protection folks I make the point that Normal <> Usual. Many contacts spend almost their entire existence in the not normal position.
I’ll see your silver lining and raise you two black clouds. - Protection Operations
![[smile]](/data/assets/smilies/smile.gif "[smile] [smile]")
