Interposing relays

[BOWILLY]

My apologies for revisiting a subject that has probably been beaten to death BUT some input please....I have had problems with PLC outputs failing various machines. The common bonds seem to be (i) load wired direct from PLC, and (ii) the loads are usually BUT not always) a solenoid valve. My old-school training - a licensed electrician for 20+ years - tells me to protect the output with either a fuse or relay. This doesn't seem to be the practice anymore, or am missing something?

 

[amberman]

Definitely practice where I am (and has been where I've worked previously and in other designs I see) to protect with fuse and relay. Frequent on/off applications we would use a solid state relay.

 

[adisharr]

I would also recommend some type of surge suppression on the solenoids.

 

[mendonsy]

I ALWAYS use interposing relays for solenoid valve loads, no exceptions.

 

[Gomez_]

Our shop does both relays and surge suppression.
On a side note, we use these relays:
http://www.wago.us/products/automation-interface-modules/current-supplies/sockets-with-miniature-switching-relay-and-solid-state-relays-857-series/overview/

 

[James Mcquade]

almost all of our outputs are triac outputs, there are some exceptions. then we use surge suppressors.

regards,
james

 

[RET]

Are you using flyback diodes?

 

[jraef]

That's a "should" that a lot of people ignore, usually for financial reasons, but do so at their own peril. I always weigh the up front cost of interposing relays against the down time cost. Takes me 5 minutes to find and replace an interposing relay, takes a lot longer to find and replace an I/O card or even an entire PLC if it's a brick style. $200 worth of interposing relays and wiring gets chewed up very fast at $1,000/minute of down time cost.

 

[psgama]

Fuses should always be installed and a flyback (freewheeling) diode should be installed across the terminals of the solenoids. Low wattage solenoids should be employed (depending on the ampacity of the PLC output) or in the case of high wattage solenoids, interposing relays installed.

 

[Phil Buchanan]

Interposing relays should be avoided if possible as they introduce another point of failure into the system but should be used if needed.

Loads such as pilot lights don't need an interposing relay but I still say fuse each output point as some choose to only fuse the entire card. This can be a physical fuse or a output card with electronic fusing.

Where possible use isolated outputs so if there is any damage it is contained.

where possible leave at least 1 open output per card so a load can be moved if needed.

Solenoids should have the proper protection applied and there are a lot of protection methods and some people have replied with their choices here but many time I see the wrong protection applied for the application at hand so make sure you are using the best method for your specific loads.

Large solenoids should be run through an interposing relay or the best method is to use low power electronic coils if possible. Electronic coils are available for most Contactors and overloads by the major players in the market today.

Brick style PLC's benefit from interposing relays across the board.

Many times people apply interposing relays where they don't need them and the customer does not keep any in stock or at least not any of that style and Bubba and Cletus can't figure out how to swap a blade type of relay and base to a pin type that they have or go from and 8 pin to 11 pin that they do stock without calling their electrical guru in which results in more downtime,etc.

So as with almost everything else in this line of work it just depends and is very application specific so there is no one hard and fast rule.

 

[Jsu0234m]

Solenoid valves have killed many output channels around here. Well water and lazy maintenance men using whatever fuse they can get their hands on have killed many outputs. Couldn't tell you how many times i've had to go swap a solenoid valve to a spare output because if the fuse blows maintenance puts another one in it that's a few amps higher until either it runs or they blow an output card instead of trying to figure out what cause the fuse to blow in the first place.

 

[Phil Buchanan]

Solenoid valves have killed many output channels around here. Well water and lazy maintenance men using whatever fuse they can get their hands on have killed many outputs. Couldn't tell you how many times i've had to go swap a solenoid valve to a spare output because if the fuse blows maintenance puts another one in it that's a few amps higher until either it runs or they blow an output card instead of trying to figure out what cause the fuse to blow in the first place.

Firing a few of the lazy maintenance men for incompetence tends to resolve that issue in my experience.

Mistakes happen and sometimes people need additional training but if you have people too lazy to do the job or just plain don't care you need to get rid of people like that because their attitude and outlook tends to spread like a virus and a workforce with lots of people that don't care will close your doors and put you out of business.

 

[gas]

PBuchanan
Many companies deserve what they get. If you fire those guys above what are you going to find to replace them with for 10 bucks an hour.
A molding plant owner I know pays 700 bucks to get his new Porsche serviced but thinks 14 bucks too high for maintenance guys. In
large plants I see one skilled guy per shift and the rest "helpers". The plant manager thinks that is adequate.
So don't blame Bubba and Cletus all the time, maybe they were thrown into something.

 

[John Morris]

IN the larger Machines, all the I's and O' are protected with circuit breakers, one amp, c scale ( fast blow ). It is a cost reduction over fuses, and reduces the need for relays.

Customers get a kick out of not having to carry the fuses in stock.

 

[sparkie]

PBuchanan
Many companies deserve what they get. If you fire those guys above what are you going to find to replace them with for 10 bucks an hour.
A molding plant owner I know pays 700 bucks to get his new Porsche serviced but thinks 14 bucks too high for maintenance guys. In
large plants I see one skilled guy per shift and the rest "helpers". The plant manager thinks that is adequate.
So don't blame Bubba and Cletus all the time, maybe they were thrown into something.

We have had this happen from management. Maintenance people have been called "too visible." and the controls team was in a lot of trouble for "not being always on the floor." The funny thing is that they spent a lot of time test benching, designing, planning, etc to implement projects management wanted done, but somehow they were just supposed to immediately know what they needed and go make it work without spending time in the office.

That being said we now have some new management that knows what they are doing and are working on changing things. I still get a crack out of the people they would put in the controls shop because they did so great on the production floor.