Ok Malador, understood. You were asking what interposing relay coil voltage should you switch through a
relay output to reduce arcing on the relay output's contact.
Malador said:
I was mainly thinking about relay outputs...
it was stated that 24 VDC powered relays were preferred for interposing relays...
wouldn't 24v AC be better, because of the zero crossing?
Sorry to reverse paraphrase your statements Malador, but I still think your a little confused about this topic.
Whether outputting to 24VAC or DC coils on interposing relays, zero crossing is not relevant to RELAY type outputs. Under normal operation, when the output goes off, the relay output will open an electro-mechanical contact and the output voltage
will be off. It does not require a zero cross as the interposing relay coil is not forming part of a circuit that needs to be driven low.
Zero crossing is only relevant when dealing with TRIAC type outputs. They are solid state outputs used for AC loads. They require the zero cross to switch the output to the low state, not off or open, just low. This is why you can still get leakage current on triac outputs and close to full high state voltage at the load. Zero crossing also has nothing to do with arcing.
So when selecting which interposing coil voltage to switch through a relay output, most select 24VDC as they usually have that power supply available in the panel. You would have to put in a transformer or suitable PSU to supply any other voltage if not already available.
Malador said:
I was also talking about using interposing relays for the purpose of carrying higher current loads...If 24V is preferred over higher voltage 120V because of reduced arcing...
With inductive loads, like relays, solonoids and contactor coils, when opening a circuit under current the voltage can spike. This creates the arcing. Without surge protection, relay contacts can pit from arcing, generate electrical noise and sometimes even weld closed.
If switching outputs from relay or transistor outputs to interposing relays, or direct to the field you should add a diode for supression across the load as close to it as possible. This prevents a high back voltage to the PLC which could potentially damage the output, or worse, the PLC itself. If using triac outputs you would add a snubber which is a resistor/capacitor(RC) supressor. The capacitor is needed to slow down the rate of rise of the voltage as the contact opens, so as to prevent the back voltage spike from firing the triac high. Also both prevent back EMF's who's resulting current could damage the PLC output. The particular interposing relays we use have this built in.
Malador said:
I've seen it multiple post where it's stated that ideally interposing relays are 24vdc...
In my opinion, to say that
most use 24VDC outputs may not be correct. What would be more correct to say is that the
most used are 24VDC outputs.
Again, it's horses for courses. At the design stage you, your company's specification, the manufacturer's specification or guidelines, or a customer specification, depending on your business, decides what voltage(s) is(are) required to switch the intended panel or field device outputs. It may be decided to use a certain coil voltage for starter contactors, another voltage for solonoid valves, another voltage for sensors and micro-switches, another voltage for indicator lamps/horns.
Obviously, as most try to use extra low voltage to keep the installation as safe as possible and where higher less safe voltages are not necessary, they use 24VDC devices as these are widely available for that reason. But many devices are 10-30VDC rated or 100-240VAC or 10-275VAD/DC, for instance. This is so that manufacturers can provide for all preferences.
In some older installations I've worked on in Ireland, there have be 24VDC, 120VAC, and 240VAC output cards, all in the same rack, driving different groups of devices, as mentioned above. No interposing relays. That was their preference.
Interposing relays were seen as a luxury as they took up too much space, extra wiring, and added cost. Nowadays, PLC's/PAC's, cards/modules and relays have become smaller, reducing their footprint in an installation and lessons have been learned on the hazards of not protecting or isolating outputs (or inputs for that matter). So now most installers use interposing relays as a default precautionary measure, regardless of whether the output load is big or small and even when the voltage is the same. Whether they realise it or not, these efforts are primarily to protect the PLC, not the cards and modules.
So the different discrete output types are transistor, triac and relay.
If an output is switching a low DC load directly, you would typically use a transistor type output. You usually get more transistor outputs per card as the circuitry is smaller than relay.
For slightly higher loads you would use a relay type output here, but they cannot handle high speed repetitive switching as they are mechanically driven and life span tends to be shorter.
If switching a low AC load, you would typically use a triac. but these are very prone to those voltage spikes. Also as they never actually drive to a safe off or open state, they can be hazardous. They can also be held on by their own load in certain circumstances, even though the PLC has switched the triac output off.
The golden rule is to keep the PLC wiring in the PLC panel. Limit the load the outputs drive, and interpose and supress those loads.
If your dealing with arcing on the contacts of the interposing relay, then use supression on their loads.
iant said:
some plc's have a limited current DC supply available(1A max)
Usually you would have a 24VDC PSU as part of your PLC chassis. This should only be used to power the PLC and its backplane, or I/O modules fed through that backplane.
If using alot of relay outputs, switching 24VDC, you would put in a seperate 5A or 10A 24VDC PSU just for this. Feed each relay output card from this PSU seperately through a breaker. Also feed each output through a fused terminal with a quick blow fuse rated for the intended load. This PSU can also be used to supply 24VDC input devices.
From my experience these methods are a good guideline, but everyone has their preferences.
G.