Mechanical Relay "Must Operate/Release Voltage" - Confusion Around Terminology

Question

I have a relay and am confused by its operation.

It is a Potter & Brumfield Relay, model R50-E2-X1-24V
Datasheet: https://www.westfloridacomponents.com/mm5/graphics/ds4/SeriesR50.pdf

My confusion stems from the "Must operate voltage" & "Must release voltage" parameters on the data sheet.
The relay in question has a nominal coil voltage of 24VDC.

The datasheet states this:

Operate Data @ 25°C
Must Operate Voltage: 75% of nominal voltage or less.
Must Release Voltage: 5% of nominal voltage or more.

Based on my interpretation of that block, at 25C, bearing in mind that my relay coil's nominal voltage is 24VDC:
(This does not make sense to me)

• the coil will operate at 18DVC or less
• the coil will release at 1.2VDC or more

I have been doing some testing with my DC bench power supply, my environment is around 26C.

I am finding that the coil will switch/operate at just below 16VDC, emitting a "small" click noise. (This seems to partially correlate with the "Must Operate Voltage" parameter, 75% of nominal or less 24VDC)
When I turn it up to just above 19VDC, there is a louder click - but nothing appears to change WRT to the coil state, nor resistance of the pole->NO pins (the pole->NC pins as well).

When I have triggered the relay, I have found that the coil will release when I decrease the voltage down to 9VDC; and it will stay released until I bring the input voltage back to above 16VDC.
To me, this does not seem to directly correlate to the "Must Release Voltage" parameter (5% of nominal or more)

My questions/confusion can be summed up in a few points:

1. What is the significance of the "or [less|more]" in the Operate Data parameters?
2. For the release voltage, 5% of 24VDC is 1.2VDC, my relay will release consistently at 9VDC. How many grains of salt am I supposed to take with the "5% of nominal voltage or more"?
3. Based on number 2, "5% or more" would mean, to me, that the coil could release anywhere below 75% (Must operate voltage) - is this a correct interpretation?
4. For the operate voltage, it says 75% of 24VDC - that is 18VDC, if I am to take the "75% of nominal or less" at face value - the "more|less" seems to be inverted to me. Should I expect it to switch anywhere between 75% or higher? (IE, 18VDC-24VDC) Or should I interpret that as switching anywhere between 5% and 75%+?

Last piece... This may be me misinterpreting the data sheet, but I would like to validate my thinking and verify if I am reading this sheet correctly or not.
I am relatively new to the electronics world, and am picking up knowledge as I go. I've just picked up "The Art of Electronics" by Horowitz & Hill, to try to understand more about this all, but please excuse me if I've misused any terms here.

Answer 1

1. The numbers are intended to guarantee the operation or non-operation of the relay over variation from unit-to-unit (but not coil temperature). They are intended to guide you in your design, to apply suitable voltages so that their relay will operate reliably.

2. You can gamble if you like. The manufacturer's guarantee is 1.2VDC at 25°C. At lower temperatures it will not be that high. In practice there is little reason to partially power a coil when it should be off, modern transistors leak negligible current, even at high temperature. Contacts can also get a bit sticky as they wear, so a marginal design can fail in the field, causing immense grief.

3. You can interpret it that way, however in practice there is a hysteresis between operate and release, as you have observed. They do not specify a "must not operate" voltage, only a "must release" voltage.

4. You must provide it with at least 18VDC if you want to operate reliably, with the coil temperature 25°C. At higher ambient temperatures you will need more voltage for the same safety margin. That includes self-heating of the coil. At 100°C coil temperature (ambient might be 70°C) you would need to give it about 23.5VDC for the same safety margin, because relays are current-operated and copper has a temperature coefficient of resistivity of about +0.4%/K.

Even if you are sure it will never be operated in a warm environment, it's unwise to operate at the lower end of the range because that sickly "click" you hear means that the contacts are not closing smartly, and unnecessary wear will likely take place. Similarly, the life specs are generally given with no flyback diode and will be degraded by the presence of a simple flyback diode.

Answer 2

"* the coil will operate at 18DVC or less

• the coil will release at 1.2VDC or more"

You have it backwards.

The relay is guaranteed to operate if the voltage is more than 75% of the 24V rating, ie 18V.

The relay is guaranteed to drop-out of the voltage is less than 5% of 24V, ie 1.2V.

Even though your particular relay may always drop out with 9V others of the same type may not, or yours may not if the temperature is different, or it was held on for many days before attempting a release or a number of other conditions.

In the region between 1.2V and 18V the relay could make contact or not, release or not. It is not guaranteed. But apply 18V and it will definitely engage, apply 1.2V and it will definitely release.