The drive circuitry in electromechanical relays is galvanically isolated from the relay contacts, and the contacts themselves are also isolated from one another. This isolation makes electromechanical relays an excellent choice for situations where galvanic isolation is required.

Now where do people keep getting the need for a opto-isolator Just asking cause in 40 years I never seen one that put line voltage on the low side.
I've fixed a bunch that was wired wrong but that's a whole new bag of worms.

 

Now where do people keep getting the need for a opto-isolator

Speaking for myself, implementation of said arrangement is by way of a second (and, as is commonly the case, third) 'line of defense' against the ΔI/ΔT inductive 'kick' attendant to switching the 'coil' -- As a practical matter the requisite 'layers of integrity' are down to the value of the 'driver-side' circuitry weighted by tolerable 'consequence and incidence' in the event of failure or malfunction thereof...

Best regards
HP

 

Well I see that point but I fix a lot of control boards almost none use opto-isolator. Most all use a simple npn to turn on the relay
And only ones I have seen fell is because of miss wiring switched hot side of line to ground but that installer error.

 

It depends on what you work on. I've got tons of industrial interface cards (in very noisy electrical environments that also use fiber links for basic communication) that use the pretty much standard 24 volts control for digital I/O. Card sends opto-isolated 24vdc current loop output signal to an distant opto-isolator LED input, card receives isolated 24vdc into a opto-isolator for the digital input. Some switch relays but we just about always use an opto-isolator as a fail-safe for high level transients or shorts to protect the embedded electronics.

http://www.portwell.com/products/IO.php

 

Well I see that point but I fix a lot of control boards almost none use opto-isolator.

I don't doubt that such failure is uncommon -- Even so, the peace of mind alone is well worth the 'hassle' of a slightly increased component count (my $.02)

Best regards
HP

 

That's where the all most none came in

industrial interface cards

I have worked with those to.
This is just for ideas I thinking more inline with lighting that is low voltage but I going to control one of two ways.
But I hear a lot about relays not being galvanically isolated which it totally no true most are for over 750 volts and more
one I replaced the most in on a GE zoneline AC unit it's rated for 1500 volts galvanically isolated.
I've fixed bad relay but maybe one or two boards got shot but that was not because the relay itself put line voltage on the low side the pcb had been shorted to the frame.

 

Well I'm thinking what the heck Ill go with the opto-isolator you talked me into it. peace of mind and .25 cent part.

 

As far as most industrial equipment I have worked with, many use a opto-isolated interface even when out putting to a relay, such as most, if not all PLC's and also the multi G4 module Opto22 boards that have been used for decades in industrial control.
Max.

 

That's where the all most none came in
I have worked with those to.
This is just for ideas I thinking more inline with lighting that is low voltage but I going to control one of two ways.
But I hear a lot about relays not being galvanically isolated which it totally no true most are for over 750 volts and more
one I replaced the most in on a GE zoneline AC unit it's rated for 1500 volts galvanically isolated.

Relays are galvanically isolated but the effective energy loop area for a transient of a given size to breach that isolation is large when compared to a typical opto like the old ild/q series used to break the ground loop. If the embedded circuit is high level and slow internally then the spikes are of little concern usually but for modern controllers in a noisy environment it's a risk not worth taking if you need long term reliability.

http://www.cypress.com/file/138636/download

 

You took what I said wrong I have not worked on as much

industrial interface cards

Just some they are the only one that I've seen with opto-isolator

That's where the all most none came in I did see some just not mess with that type of boards.