# Some question about a relay data-sheet

I want to use this(data-sheet) electromagnetic relay given this pin-out:

But from its data-sheet I couldn't figure out whether it is normally open or normally closed. It doesn't write it explicitly.

So I have some questions regarding the data-sheet:

1-) Is this a normally open relay? What indicates that?

2-) There are also the following parameters:

What is limiting continuous current? Does that mean the relay will open if the continuous current exceeds 6A? I also don't underrated what it means for 10A in parenthesis. All 13 14 and BB bridged?? The sentence is not so clear.

3-) How come can this coil draw only 18mA at DC. I though coils have high resistance for AC voltages? Does that mean coil's DC resistance is 1.3k? Data-sheet also does not mention any peak current for when the coil is de-energized. I thought the coil will have some high current during entering to off state.

4-) Can I use this relay for application rated for nominal 10A?

Right there on the front page it says: -

PLC relay, consisting of base terminal block PLC-BSC.../1 IC/ACT with screw connection and pluggable miniature relay, for high inrush currents, for assembly on DIN rail NS 35/7.5, 1 N/O contact, input voltage 24 V DC, max. inrush current up to 130 A

And....

What is limiting continuous current? Does that mean the relay will open if the continuous current exceeds 6A? I also don't underrated what it means for 10A in parenthesis. All 13 14 and BB bridged?? The sentence is not so clear.

The current is the continuous maximum current the contact is rated to take and, if you use all the terminals available for that contact, the continuous maximum current can be regarded as 10 amps.

Can I use this relay for application rated for nominal 10A?

I would say that would be taking a risk that is too great.

How come can this coil draw only 18mA at DC. I though coils have high resistance for AC voltages? Does that mean coil's DC resistance is 1.3k? Data-sheet also does not mention any peak current for when the coil is de-energized. I thought the coil will have some high current during entering to off state.

24 volts and 18 mA is a coil power of 432 mW and sounds around about the right sort of level that you can expect froma relay of this type. Yes it will have a DC resistance of 1k333.

The peak current when the coil is disconnected will be 18 mA and you need to rate any back-emf protection devices for this current (not really a big deal of course). If you don't use a protection method then you might get a peak of thousands of volts that could destroy your transistor.

• Sorry mate Im not that into these. I didnt notice N/O Commented May 29, 2018 at 11:21
• Do you think that protection is built-in for this module? back emf i mean Commented May 29, 2018 at 11:34
• In data sheet it says "Protective circuit Protection against polarity reversal Polarity protection diode Free-wheeling diode Damping diode" Is that what you mentioning? Commented May 29, 2018 at 11:39
• Looking at the data sheet again I would say that the final diagram shows / implies an internal diode and this, coupled with the words must mean it has a back-emf protection device built in. Yes, that is what I was saying in my answer. Commented May 29, 2018 at 12:08

What is limiting continuous current? Does that mean the relay will open if the continuous current exceeds 6A? I also don't underrated what it means for 10A in parenthesis. All 13 14 and BB bridged?? The sentence is not so clear.

It means that the contacts will overheat and cause related hazards (melting, fire, etc.). Bridging the contacts allows for higher current.

How come can this coil draw only 18mA at DC. I though coils have high resistance for AC voltages? Does that mean coil's DC resistance is 1.3k? Data-sheet also does not mention any peak current for when the coil is de-energized. I thought the coil will have some high current during entering to off state.

It is a DC coil, there is a reverse polarity protection and a freewheeling diode built in. Also it is meant for continuous powerso there is enough resistance to avoid overheating.

So yes you can use this relay for 10A load however it is not a bad idea to look for a higher rated relay if you can get them. Especially one where you don't need to bridge the contacts.

There is zero information on relay life in that datasheet**. I would not use it at all without proper information.

** yes, there is a "mechanical life" spec but that just tells you how many times it will clack back and forth (switching zero current) before it falls apart. What you need is the life when switching the type of load you intend to switch. That "electrical life" may be as brief as 10^4 operations rather than the 10^7 a naive optimist might infer from the datasheet. You would have to plug in 1,000 replacement relays to get to the 10^7. (If you exceed ratings such as peak inrush current then it may be even more brief).

The 10A/6A dichotomy appears to me to be related to overheating of the terminal block/socket/relay pins rather than the plug-in relay contacts.