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Comments
gbradley | 08 Apr, 2004 12:00 AM
Excellent!
Posted: 13 years 7 months ago   | Permalink
Dave Shutters | 16 May, 2004 12:00 AM
Great article. Should be required reading for every controls designer.
Posted: 13 years 6 months ago   | Permalink
Dulon | 30 Jun, 2004 12:00 AM
Just excellent! I loved to read such article on electrical safety.
Posted: 13 years 5 months ago   | Permalink
Pierre | 19 Aug, 2004 12:00 AM
That stuff should be read to all soon-to-be EE. I've been in this for 20+ years and am amazed by the clarity and real-life aspect of this text. Bravo Jim!
Posted: 13 years 3 months ago   | Permalink
steve weisz | 18 Jun, 2005 12:00 AM
Great articale. This will be very usefull in real life aplication around the industry. excellent job.
Posted: 12 years 5 months ago   | Permalink
Eddie Hill | 23 Jun, 2005 12:00 AM
Great article! This needs to be read by ALL control designer, integrater, electrical engineer, professor, technical...ect.
Posted: 12 years 5 months ago   | Permalink
Big Country | 22 Nov, 2005 12:00 AM
Great article, really shed some light on things for me. You can never be to safe or take to any precautions.
Posted: 12 years ago   | Permalink
Jiger Jim | 19 Mar, 2006 12:00 AM
Super article. I only wish that the previous guys who designed/maintained the plant at my new workplace had read it!
Posted: 11 years 8 months ago   | Permalink
unni | 21 Apr, 2006 12:00 AM
Excellent
Posted: 11 years 7 months ago   | Permalink
Keith | 08 Feb, 2007 12:00 AM
Great article but I would like to make one observation about point 9 (Use ELV DC for control field wiring.) I have found several times when using ELV DC for wiring field switches everything works fine for several years but after a while, switch contacts begin to erode and the circuit will start failing to operate intermittantly. On looking closely at the contacts, a peak can be observed on one of the contacts and a matching pit on the other contact. Perhaps this is only when inexpensive swithces are used that are designed for LV and higher currents than the milliamp controls used in the typical PLC inputs. I would be intrested in knowing if others have found this to be a problem. I don't know of any way to get around using low currint DC inputs on PLC input circuits.
Posted: 10 years 9 months ago   | Permalink
eoates | 18 Feb, 2007 12:00 AM
Great article. Just a note of caution, however:
It's just convention that the negative side of the power supply is usually grounded. If the Positive side of the Power Supply is grounded instead, then it's the other way around. In the case of positive ground, you would WANT to use NPN (sinking)input devices and input cards that source current. Maybe I'm dating myself, but the old Fords from the 1950's had a 6 VDC positive ground system.
Posted: 10 years 9 months ago   | Permalink
fredrick | 04 Mar, 2007 12:00 AM
finally some one said it! I'm surprised that all the things i have seen trough the years have finally been put together in one article ... and with the appropriate tone ... thanks JIM
Posted: 10 years 9 months ago   | Permalink
Gilad Gozani | 07 Dec, 2009 12:00 AM
Sorry not to accept things as shown here!
* FIRST, grounding the control voltage is not a must. Instead, I prefer the ""filter"" technique where either the negative or the positive is connected to ground via parallel resistor (2-4K) and a capacitor (1uF). This ""filter"" removes any hazard of charges accumulating on the rails and eliminates AC that might be picked up by interferences (AC noises might be affecting capacitive sensors and other sensitive devices). This ""filter"" adds to the safety stated by the article in the way that if ""live"" POSITIVE (or NEGATIVE) shorts to ground, no harm is caused.
* SECOND, NPN and PNP are symmetrical as said earlier by eoates on 2007-02-18. If you must ground one rail, it should be either the negative one, for NPN, or the positive one, for PNP.
Some History: N and P layer are the building blocks of electronic circuits. N allows negative charge travel (electrons) while P enables positive charge travel (holes). Since electrons are much smaller, they travel 3 times better. For this reason, NPN transistors are far better and cheaper then PNP transistors. This is why in the past sensors mostly used NPN outputs, later, PNP became available. Today's electronics has long moved to mosfet's and for the same physical reason most power outputs use N channel transistors (negative switch). Usually thanks to ""charge pumps"", they can serve as positive switches as well producing the same efficiency.
May we all keep producing safe equipment wisely.
Posted: 8 years ago   | Permalink
ControlsGirl | 07 Aug, 2012 12:00 AM
Two years into the industry and I haven't had anyone spell these things out for me. Great reference! p.s. I'd love all the advice I can get. If you happen to be a great engineer and wouldn't mind sharing your secrets and helping me with advice, I'd love to have your email address.
Posted: 5 years 4 months ago   | Permalink

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