Late Break and Early Make

I hope the triviality of this question doesn't betray my experience level, but I'm just starting to get into safety system design, and this question, while always bouncing around in the back of my head, has now taken on some significance.

I understand what normally-closed late-break (NCLB) and normally-open early-make (NOEM) mean, but why do we care? What do these particular distinctions accomplish that make them so much safer than standard NC and NO contacts?

Thanks.

P.S. For those who may never have heard:

NCLB contacts remain closed until the operator is fully mechanically latched. For example, when you push an e-stop pushbutton, the contacts stay closed until the pushbutton reaches the fully pressed position.

Conversely, NOEM contacts close BEFORE the operator is fully mechanically latched. So, when pulling an e-stop pushbutton back out after pressing it, the normally-open contacts will close BEFORE the button has reached its fully pulled position.

recondaddy said:

So, when pulling an e-stop pushbutton back out after pressing it, the normally-open contacts will close BEFORE the button has reached its fully pulled position.

Click to expand...

... then you press the "Reset E/stop" pushbutton to reset the safety controller, if the channels are happy the unit will reset, if it isn't happy it will not!

Surely you're not relying on just pulling out the e/stop !

.

Silva,

Of course I would never rely on just pulling out the e-stop. I recognize that in order to be compliant with all safety guidelines, the safety circuit must be closed AND reset -- not just closed.

My question only has to do with the importance of NCLB vs. NC and NOEM vs. NO. Why are NCLB and NOEM contacts preferred over ordinary NC and NO when it comes to safety systems?

I am not at all a safety engineer or up to speed on safety systems but, elsewhere in control circuitry Late Break or Make before Break means a transfer contact (probably Form C but possibly others) that, during mechanical transition, establishes a contact with the formerly open pole BEFORE releasing the contact with the formerly closed pole. Early Break or Break before Make is the usual pattern of the common having no contact with either pole in the middle of transition.

I suppose there are lots of reasons for Early Make but usually, in my experience, the common had some kind of hardware on it that would generate an unacceptable arc or transient if left to float, even for a few milliseconds. Making before breaking would keep the circuit under control so it could not flash or ring. Of course, you had to design the circuits on the open and close pole so they could be shorted together for a short time without bad things happening.

Just thought of another use for Early Make. The circuit on the switch common involved latching contacts for a relay or contactor where, if the circuit floated momentarily, the latch would release.

Oldtime relay engineers could probably fill these pages with other examples of unusual contacts and relay operations. From what I've seen in the railroad signaling industry, they take it to the most extreme!

"I suppose there are lots of reasons for Early Make but usually, in my experience, the common had some kind of hardware on it that would generate an unacceptable arc or transient if left to float, even for a few milliseconds. Making before breaking would keep the circuit under control so it could not flash or ring. Of course, you had to design the circuits on the open and close pole so they could be shorted together for a short time without bad things happening."

Click to expand...

Thats the way I see it too. For example you want to stop a VFD and also open the contactor. I guess the generalization will be when racing two signals for machine safety and operator safety.

Think of a current transformer. You NEVER open the secondary. Now if you are switching to read current on phases A B and C you have to short the A CT before you open the contacts on the A meter. On B phase you would want to connect the meter (early make) before breaking the CT shorting contact.

I am not certain why you would need this on an E stop pushbutton type switch. Especially so when one of the requirements is the E stops are not allowed to reset a circuit by themselves - it takes a separate action.

Dan Bentler