# Emergency stop



## jbl (Mar 24, 2017)

I have a question for a equipment. The emergency stop is connected in serie with the primary of transformer. The transformer is 240V, 1500VA. It's a Techspan button. The contact it's indicated 240V, 10A but the other side AC-15, 240V, 3A. What information is right? The contact of emergency is ok for the transformer (1500VA/240V=6.25A)??


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## MechanicalDVR (Dec 29, 2007)

I'd go with the 3amp rating.

*BUT*

That sounds crazy to me!

Normally the E-stop is only tied to a shunt trip breaker.


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## Helmut (May 7, 2014)

I don't think I've ever seen an E-stop controlling a transformer primary...ever.

If anything, it's downstream of it.


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## MechanicalDVR (Dec 29, 2007)

Helmut said:


> I don't think I've ever seen an E-stop controlling a transformer primary...ever.
> 
> If anything, it's downstream of it.


Me either, very strange indeed!


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## jbl (Mar 24, 2017)

But the contact support 10A.


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## 460 Delta (May 9, 2018)

Are the contacts momentary or maintained as in locking?
Killing the control power will get it done I suppose but if you have a starter contact weld or hang, you’ll want a shunt trip breaker or something similar.


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## AK_sparky (Aug 13, 2013)

I'd guess that the 10A rating is for AC-1 category, which is non-inductive loads.

The 3A rating is most definitely for AC-15 category loads, which are electromagnetic loads.

https://en.wikipedia.org/wiki/Utilization_categories


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## macmikeman (Jan 23, 2007)

If you search around here you will find references to welded contacts , though not normally happening to control contacts I don't think. Man oh man if an emergency stop doesn't stop something though in an emergency...... Seems a strange setup.


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## paulengr (Oct 8, 2017)

Welded contacts do happen in controls. It's kind of a strange thing that happens once in a while but it's kind of unpredictable. We can't 100% eliminate it, just make the contacts oversized so it happens less often. Safety relays have contacts that are grossly oversized for the job to lower the odds down to basically 1 in a million (6 sigma).

The transformer is an inductive load so you go by the inductive or AC-15 rating, not the resistive or AC-1 rating. An inductive load especially a transformer has a big current surge when it energizes and a voltage surge when it de-energizes because of the magnetic field of the coil so the contacts have to be heavier to absorb this. If you don't the switch will burn up prematurely or as mentioned weld together and never open again.

A better way is to install a small 45 mm contactor rated for at least 7 A (nearest standard size will be 9 A) with a 240 V coil and wire up the standard 3 wire two push button start/stop circuit except it will be labelled "E-Stop" and "Reset". That also meets the NFPA 79 standard that an E-Stop button has only one function instead of two, and meets the OSHA hydraulic press and woodworking tool anti-restart on power loss relay regulation. The cost for the three parts is still less than the labor to put it in.

There is nothing wrong with standard push buttons for E-Stop contrary to popular belief. Under the machine control standard (NFPA 79) the button should be a red mushroom head with a yellow background on the panel. It is triggered by a human being and in safety standards under the best conditions human performance has an error rate of 10% aka "SIL 1". Thats also what standard nonsafety industrial controls are rated so non-safety rated controls are SIL 1 by definition. You could put a fancy safety relay or button there with better reliability but it doesn't matter if the button only fails once in a thousand years if the monkey pressing it misses way more often. By the way the 10% error rate assumes calm, cool, collected conditions...in other words what the guy doing QC inspections does, never the case for E-Stops. Under emergency conditions (which is why it's an E-Stop), human error rates have been shown to be as bad as 40% in test after test done by military and chemical plants. Usually that's because freeze/flight kicks in and the operators just stand there repeating something like "oh no" over and over in total panic usually drooling or pissing all over themselves, or jump like jack rabbits and head for the hills. This all happens in seconds when the cerebellum which is the little tiny animal brain at the back of your skull takes over while the frontal cortex is still thinking about where to go after work and why there is black some coming out of the panel. By the time the cerebellum shuts down the cortex is just starting to think about all the blood everywhere and whether or not this will mess with plans after work. Very few people are cool as a cucumber and calmly reach over to hit the E-Stop, which is also why they should never be relied on as a safety system in an emergency! You might be laughing when you read this but I'm certified in accident investigations, and safety system designs and analysis. On Monday this week after vacation I came back to fix a 4160 starter. It didn't start right and it sounded like the motor was going to fly apart. The mechanic made like greased lightning. I tried the controls first. The Scada was wired to bypass all the buttons so no luck there. I pulled the disconnect which jammed halfway which many do but not sure if this was one of them or it would just energize so I was stuck holding a disconnect with a starter acting up and controls that won't stop, waiting for the mechanic to stop cowering and get curious enough to come back. Once the Scada stopped sending a run command I could release the handle and do damage assessment.

Sent from my SM-T350 using Tapatalk


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