stop circuit regs

[bigheadted]

i am currently working on a macine that is modular and selectable, it has both software and hardware estop ciruits. because of its selectiblity this circuit is extremely complicated for a hardware stop circuit. i would like to put some sort of indication in to help identify any breaks in the hardware stop circuit in the event of a failure. is this within health and safety regs, i was thinking of using leds between the estop loop and neutral to prove the presence of 24v at various parts of the estop loop circuit.
is there any other inline devices that i could use for indication
thanks

 

[waross]

athere is no problem in a standard motor control circuit. However if your circuit is a non standard circuit we can't say with certainty. Also, many E-stops are momentary devices and the contacts close again when you release the pressure on the button.
Many machines with complex control systems use a master control relay. The E-stop drops out the master control relay which de-energizes everything until the relay is reset. It is no problem running a light in parallel with the relay coil or using normally closed contacts to indicate "No control Voltage"
As for recomending the addition of an indicator light into a system that mixes hardware and software controls, I would be concerned that the indicator may pass enough current to activate a sensitive logic input. I would have to see the circuit drawings and possibly the specifications on the various pieces of equipment in the circuit.
An option that I do feel comfortable recomending is to use the software to monitor the E-stop and generate a software indication of E-stop operation.
yours

 

[bigheadted]

thanks waross
i perhaps should have expained further that this is circuit that maybe contains 200 or so estops that are divided up into indivdual groups of maybe 10 that are selectable between two master control stations each having a master stop relay as you described. each stop is currently connected to a plc which indicated to the opperators via a graphical display which stop has been pressed so the logic side of things is not an issue because this is caterd for.
i am trying to make it easier to find a break in the hardware loop in the event that it is broken in a failure rather than normal opperating conditions sy that a softwre contact was opperating fine on a sop but the hardware had failed. i was thinking that this might involve mutiple indicators in parallel with the stop relay at various points in he circuit to indicate the location of the break
cheers rich

 

[waross]

Hi bigheadted
E-stops are usually normally closed. most failures are "failled open" so that a broken wire will have the same effect as a stop signal. A broken wire to a stop button usually has the same effect as an emergency stop and usually will give the same indication on the operators display as an operated button.
There are a few notable exceptions such as cranes and moving parts with electrical devices attached, but in the overwhelming majority of control schemes, broken wires are not an issue. Annunciation of devices for trouble shooting is done to identify which device caused a shutdown, not to find broken wires. This can be accomplished in the software.
respectfully

 

[jraef]

From a logic standpoint though, can you add a Normally Open contact to each E-stop device and use them to latch a relay if operated, then compare them to the E-Stop circuit? If no E-Stop device has been operated but the E-Stop circuit is open, there must then be a wire break.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ScottyUK]

You could split the large circuit into a number of zones, each zone being connected to a normally-energised relay. From each zone relay connect a N.O. contact (i.e. closed when the relay is in the energised state) into a master trip & lockout relay. If you use a relay equipped with an LED indicator for the zone relays this would assist with diagnostics, or use an auxiliary contact on the zone relay as a repeater to the control system.

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I don't suffer from insanity. I enjoy it...

 

[dbaird]

NFPA 79:10.7.2.2

"Push button-type devices for emergency stop shall be of the self-latching type and shall have positive (direct)opening operation."

Therefore they must be normally-closed maintained contact type that requires manual reset/release. We use an illuminated "push-to-stop / pull-to-release" style so that if several are wired in series around a machine or cell it is easy to detect which one was pushed.

As for the software/hardware issue please go to the following Square-D link to open and review pertinent standards/requirements presented in Appendix C pages 319-322 of their Safety Catalog:

http://ecatalog.squared.com/techlib/docdetail.cfm?oid=0900892680169d88

David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.