4

On electrical schematics, is there a standard for whether there is a space between the numerical value and unit symbol? I can find standards that require it in general writing, but none that specifically address it for electrical schematics. Sometimes space is at a premium, e.g. 5A might fit within a circuit breaker symbol where 15 A would not, so that's the reason I could see that spaces are not required in schematics.

NIST succinctly summarizes the requirement for writing, which comes from section 7.2 of the NIST Guide for the Use of the International System of Units (SI) states

There is a space between the numerical value and unit symbol...

The International System of Units (SI) prescribes in section 5.4.3:

The numerical value always precedes the unit and a space is always used to separate the unit from the number.

ISO 1219-2:2012 for pneumatic/hydraulic schematics uses a space in almost all their examples, but page 39 does also show 0.03cm³/stroke without spaces.

I have access to and checked UL 489 (Apr 22, 2019) and UL 508A (Apr 24, 2018), but they do not address units directly and do not have example schematics, either.

Edit: And our own Electrical Engineering Stack Exchange Rules and guidelines for drawing good schematics also omit spaces, but those examples are for PCBs rather than electrical panels which is what I'm looking at.

Micah Lindstrom
  • 143
  • 6

1 Answers1

7

ANSI/IEEE Std 280-1995, "IEEE Standard Letter Symbols for Quantities Used", chapter 3.4 states

In the complete expression for a quantity, a space should be allowed between the numerical value and the unit symbol.

They use should which means it is encouraged, but not necessary. In their example, you should write 35 mm, not 35mm.

The rules I go by:

  • In schematics, I omit spaces between number and unit to save space. I figure schematics don't need to be grammatically correct. I adhere to symbols and and reference designation letters put forth in IEEE Std 315.
  • When writing reports, I put a space between the number and unit, adhering to the recommendation in Std 280, chapter 3.4.
qrk
  • 7,474
  • 1
  • 5
  • 20
  • 2
    Glanced over some of our production/released schematics, and notations seem to be all over the place. I've seen 10K and 0.1 ohms for resistor values, 200pF and 1 uF for cap values, and 10V and 50 V for a cap's working voltage. – SteveSh Nov 19 '21 at 02:02
  • 1
    It seems the answer is "No, there is no standard for schematics". This answer is as good as any, so I'm accepting it until/unless something better appears. – Micah Lindstrom Nov 22 '21 at 17:03