Machine safety relay or not?

[gerard]

It comes down to risk and fail safe, i do build controls without safety relays but i implement a e-stop that will open circuit if broken off, also i use 2 no# contact blocks in series, i also have independant relays working off this; one to drop out main contactor and one to open circuit the drives control. In this setup both relays would have to fail and the e-stop fail on both contact blocks, and as added protection running the retaining circuit for the main contactor through the e-stop too independantly give a system thats dosnt require safety relays. Also to remember is if a drive is fitted you will be required to fit some form of contactor upstream to give electrical isolation thus reducing the risks of capacitive shocks if the unit is turned off or a power cut occurs.

I was told by Inverter manufacturer that the safest way to carry out this is to use the safety relay to enable the drive and energise a contactor at the Inverter output. This is as close to fail-safe as I can get it - unless I put a box over the machine!

 

[Darkwood]

I was told by Inverter manufacturer that the safest way to carry out this is to use the safety relay to enable the drive and energise a contactor at the Inverter output. This is as close to fail-safe as I can get it - unless I put a box over the machine!

The advice you are given is one method and opposing other responses it isnt really good to crash power on an inverter even on e-stop hence they suggested a contactor on the out-going side as crashing power can corrupt the software, some inverters can have their enable signal opened a fraction quicker than the power relay drops which allows controlled shutdown this can be a option found on selected safety relays to have a biased channel when opening circuit, there are also drives on the market that can use their enable(run) function as e-stop too and meet requirements.

 

[gerard]

I have ordered a SICK safety inductive sensor and safety relay for this application and am just waiting for them to come in. I heard AB drives have a E/Stop control input - but I only found this out after I had installed an Emerson drive - live and learn i suppose! But I am going to use the relay to control enable signal and contactor as I fell it is the best option at this stage and the easiest to implement.

 

[pmac10]

Gerard, i think Darkwood is referring to enable/run as one input which is the case for most inverters, but you mention an Emerson drive where enable and run are two different inputs so what you're doing is perfectly safe. ive been advised in the past by emerson to wire an e-stop to the enable input but have seen enable just looped to run.

 

[gerard]

Gerard, i think Darkwood is referring to enable/run as one input which is the case for most inverters, but you mention an Emerson drive where enable and run are two different inputs so what you're doing is perfectly safe. ive been advised in the past by emerson to wire an e-stop to the enable input but have seen enable just looped to run.

Yeah, I am pretty confident this is the best way to go about it. I was talking to an engineer and he reckons a the AB drives are the only drives that the enable can be used as an e/s.

 

[netblindpaul]

gerard,
He is wrong, there are others.

 

[Darkwood]

This subject of e-stopping a drive has got many a forum scratching their heads, firstly you need to evaluate what exactly you require the motor to do in an e-stop without the obvious STOP IT! been suggested, is it safer to shut power off and let it coast or will a controlled decel' DC injection be more practical for example.
As ive mentioned before dropping power off upstream of a VSD should be considered a no-no as a no' of issues can occur from a longer coast to stop time as the drive dumps stored energy into the motor to drive corruption, also if fitting a contactor to the output side some drives can be left in a ramp up state so simply re-engaging the contactor could give instant motor run as the drive has only gone into a motor blind state. You should then try to employ the practice of both having an 'Enable/run' function open circuit as well as a contactor between drive and motor. A chat with the drive manufacturers may be required as using Enabled/run function will in most cases allow a safe E-Stop function but few drive have been designed around this and thus dont comply with current standards when using this setup for E-stop as issues of fail-safe etc have to be considered, ABB are one brand that do comply in some of their models as mentioned before and im sure others are out there too, all in all their has to be alot of thought put into the control system, a risk assessment of how the Motor is required to stop whether its safer this way or that etc etc .....

 

[gerard]

gerard,
He is wrong, there are others.

Ok - well that must be the only type he knows of. At the moment i have an Emerson installed but if I am carrying out a similar application in the future, I will look for Inverter with the enable safety feature.

 

[gerard]

I am using the enable and run signal to stop the drive at the moment so that's pretty safe. As I said, I am waiting on a safety relay and sensor. I believe I am going above the original standard of safety set by manufacturer. The reason for my concern is due to the fact that I altered the machine and I want to be satisfied that I am doing my utmost to make it as safe as possible.