Surely the way it is written means that it is only 'guaranteed' to work between 27.6V and 195.5V.
No, as said already ... 24-230v plus or minus 15% is not 27.6v and 195.5v
True, but we are trying to make some sense of the way they've expressed it. It can be argued that one could be excused for assuming that "24V - 230V ±15%" meant:
"The lowest controlled voltage guaranteed to work is 24V ±15% and the highest voltage at which it is guaranteed to work (satisfactorily) is 230V ±15%."
...which would lead to EFLI's interpretation that it is only guaranteed to work with control voltages between 27.6V and 195.5V.
+/- 15% means that the device will operate within 15% either side of the expressed voltages.....
That's another interpretation. It's probably the one which, in context, makes the most sense, but that doesn't make it a satisfactory (or unambiguous) mathematical practice.
its a mathematical form of expressing standard deviation/tolerance. It would be no different when a machinist gets an engineering drawing from a designer to make something on a milling machine. The drawing may state measurements in millimeters +/- 0.05mm. So this means that the final dimensions of the workpiece can be 0.05mm over or under the spec, but will be within the tolerance.
As I keep saying, a spec may be expressed as a range of acceptable values
OR as a nominal value plus an acceptable tolerance around that nominal value - but if you try to do both at once, you simply add confusion and ambiguity...
... it would be conventional for a drawing to say that the acceptable final dimension of a part is "4mm ± 0.05mm" - which is clear and unambiguous. One could alternatively say that the acceptable final dimension was "3.95mm - 4.05mm" - which is equally clear and unambiguous. However, what one earth would one make of, say, "3.96mm - 4.04mm ±20%"? .. and, more to the point, why on earth would anyone use such a contorted and ambiguous method of expressing something which could be expressed clearly in either of the ways mentioned above?
Its the same with the control voltage on our relays - the control voltage is stated as requiring to be 24-240v +/- 15% so the control voltage can be 15% under the minimum stated and 15% over the maximum stated. As a bonus, it will also work with any voltage between those 2 figures....
I understand what you're saying, but it still doesn't make any mathematical (or other) sense. If the spec says that the relay is guaranteed to work at 15% under 24V (i.e. 20.4V), at 15% above 230V (i.e. 264.5V), or at any voltage in between, then why not just say that it is guaranteed to work between 20.4V and 264.5V? How do voltages between 20.4 and 24V, or between 230V and 264.5V, differ from those between 24V and 230V, if it is guaranteed to work at any of those voltages?
Kind Regards, John