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Machine safety relay or not?

Discuss Machine safety relay or not? in the Commercial Electrical Talk area at ElectriciansForums.net

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gerard

Regular EF Member
Hi,
I have an industrial mixer at work that used to be powered by a dual speed motor. These particular motors are expensive and not always easy to get off the shelf in the event of a breakdown. I have replaced this a 4 pole single speed motor, controlled by an Inverter. The old motor was controlled with contactors and these contactors were in turn controlled by the machines PCB circuitry.
The motor drives a spiral - this spiral will only operate when the mixing bowl is in place and is no potential harm to an operator. When the mixing bowl is taken away by the operator, the spiral is exposed. Although the is minimal risk of harm, I would like to eliminate any risk whatsoever. I am going to install an inductive sensor that will detect when the bowl is in place, this sensor will then be used to enable the VFD to drive the motor. There are a few other conditions that have to be met for the machine to start so my extra sensor is not the only safety device. Would this be a suitable method of safely disabling the machine or is there a need for a safety relay? Has anybody any experience/suggestions on this? Is disabling a drive with a control signal acceptable or would I need a contactor to cut power to drive, hence stopping the motor?

Cheers
 
K

Knobhead

If you think about it any E-Stop circuit is a control circuit, how else do you do it? An E-Stop P/B with 100A 3Ph contact blocks at the back? You have to design so that any failure is “fail safe”.
You know the details of the system, it’s up to you to use your noodle to make it the best you can. A main contactor for the machine is a good idea. Paul will probably shoot me down.
 

silva.foxx

Electrician's Arms
I recently worked on a spiral mixer, one where the bowl is hydraulically clamped before the hood is hydraulically lowered before mixing commences. It was a manufacture of three letters.

It was a relatively new piece of kit. Anyway, that didn't have a safety relay but the bowl had to be secure, and hood down before it could start... perhaps this is enough to satisfy that risk assessment.

Your RA is what should tell you what's satisfactory. If you can move your bowl or lift the hood whilst the spiral is turning then you will need to eliminate the risk. How will your 'inductive sensor' know that a bowl is in place, as opposed to some other form of metal ie. wedding band, metallic tape, any other operator utensil?
 

gerard

Regular EF Member
I recently worked on a spiral mixer, one where the bowl is hydraulically clamped before the hood is hydraulically lowered before mixing commences. It was a manufacture of three letters.

It was a relatively new piece of kit. Anyway, that didn't have a safety relay but the bowl had to be secure, and hood down before it could start... perhaps this is enough to satisfy that risk assessment.

Your RA is what should tell you what's satisfactory. If you can move your bowl or lift the hood whilst the spiral is turning then you will need to eliminate the risk. How will your 'inductive sensor' know that a bowl is in place, as opposed to some other form of metal ie. wedding band, metallic tape, any other operator utensil?
This works the exact same way - I tried using an inductive sensor that was already there to detect the presence of the bowl, but it was incompatible with the Inverter. So this is why I want to add a new sensor in to enable the drive. That's fair enough I will add the enabling sensor for my own piece of mind - it's always nice to get your thoughts on it though!
 
M

malcoa

The sensor is fine as an additional feature to prevent the drive from getting its start signal but would not suffice as a safety device to prevent injury, as you probably know it would need to be a 2 channel device, does the machine need to be powered up while the bowl is removed? Either way the owner should in addition put adequate procedures in place to prevent injury such as isolation, there is nothing wrong or bad with your idea at all, its the context that might be questionable when you put the word safety into the equation.
 

netblindpaul

Forum Mentor
There could be many things wrong with the suggestions.
A drive should never have its inhibit from a sensor.
The drive unless "SIL3" should never be "allowed" to act as a safety device.
Contactors are better.
Is the machine CE marked?
If it is and you modify it, then it is doubtful that you will ever comply with PUWER98 again wrt this machine without expert guidance.
As the designer of the mod I hope you can prove your competence to do this in a Court of Law when it comes to it as your Employer will be at pains to put you out on your own unless this is very explicitly specified.
 

gerard

Regular EF Member
There is a lot to take in there. Ok, so a sensor doesn't count as a safety device but if anything, I am improving on what the manufacturer has done. There is an e/stop that will stop the mixer (as per manufacturer spec) and I want to add the sensor to enable the spiral inverter when the bowl is in place. The only modification I made to the mixer is add an Inverter. So do you think I should go down the safety relay and contactor route? Also, how does one evaluate the risk involved in a situation like this.
 
P

pmac10

i would use a safety interlock switch rather than a sensor and include this as part of a loop on your safety relay, i have worked on similar sounding mixers recently where limit switches were used to detect bowl presence but washdowns/corrosion could cause them to stick.
 

gerard

Regular EF Member
I'm going to agree and add the safety relay. I will then use this relay to control a contactor switching the motor - surely this will be sufficient?
 
M

malcoa

if you add the safety relay, you should use safety switches rather than sensors as they are 2 channel and fit for purpose, if the machine is CE marked it should have enough in the way safety and adequate control circuits for it to function and to prevent injury risk, its ok to swap the contactors for an inverter but is do you need to alter the controls and safety circuits to such an extent
 

gerard

Regular EF Member
So if I get a safety sensor/switch and a safety relay and use this to disable the drive, you reckon this will be suitable - no need for contactor?
 
K

Knobhead

I would use a contactor and not rely on zero O/P from the inverter.
 

netblindpaul

Forum Mentor
You should not switch the output of the inverter with a contactor unless the maker says this is OK.
You should not really use a safety relay to directly switch any load unless the maker says its OK.
You really need to look a EN 954, & EN 16849 along with EN 60204-1.
you must comply with the requirements of PUWER98 & EAWR89.
The ACoP on PUWER will guide you to 60204-1.
This will guide you to a raft of other standards to ensure compliance, thus to comply with PUWER you must comply with these standards.
If the machine is CE marked then as you are changing SRPCS then you must re-do the design RA, the design FMEA and a raft of other things to comply with statute law.
Even in fitting the inverter & changing the motor you must do this.
 

darkwood

Super Moderator
Staff member
Super Moderator
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.
 

gerard

Regular EF Member
I am told to not compromise on safety at all costs. this means I am going to put a safety relay in the machine. I have a guy from Sick calling round on Tuesday and he reckons there are safety sensors on the market - hopefully this will do the job!
 
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