Let two different people draw the same circuit. One draws the contactor this way, the other that. One puts the fuse cap up, the other to the side. A third person looking at the two pages has to work out what each shape means before he can tell they are the same circuit.
This is not a matter of visual preference. A drawing is a tool that has to be readable independently of its author. The meaning of a drawing changing when its author changes is against the purpose of the tool. Everyone who draws symbols by hand, without noticing, draws his own symbol set along with them; as the office grows, those symbol sets drift apart from each other.
The hidden cost of hand-drawn symbols
The first cost is reading. A fuse drawn by an intern is recognised by someone who did not draw it, but not with certainty. The question "is this a fuse or a thermal?" is asked again in every new drawing.
The second cost is the learning curve. The newly arrived engineer first has to learn the office's symbol set: how a contactor is drawn here, how a power line is shown here. That set is not written down, so the way to learn it is by asking; if the person to ask is not available, the drawing falls into a waiting queue.
The third cost is continuity. The drawing of a person who has left the company is a memory of that person's symbols. For someone else to touch that drawing starts with reading that memory.
A symbol is more than a picture
At this point it is worth making clear what a symbol is. A symbol in a drawing is not just a picture. That symbol has pins: how many connection points, with what name, at what position. The coil terminals of a contactor, its main contact terminals, its auxiliary contact terminals are separately defined.
A shape without pins is a sketch placed on the page. A symbol with pins is a thing the circuit connects to. Cables are tied to pins; addresses fall on pins. The reason EupCAD's rule check can give an "unconnected pin" finding is this: the check looks at whether the defined pins of every symbol are actually connected. That check cannot be done on something that is a picture.
IEC 60617: a common language
IEC 60617 standardises the symbols in electrical schematics internationally. How a contactor is drawn, how a relay, a thermal fuse, an earth is shown, is in the domain of this standard. The aim is that a drawing is readable independently of its author; a contactor drawn to the standard is recognised by everyone who knows the standard.
EupCAD's built-in library is 802 symbols prepared to this standard. When you put a contactor, a relay, a fuse, terminals, power symbols on the page, these are the shapes from the standard. They are found by typing a name, a category or a letter into the search bar; whoever types "contactor", in whatever office, brings up the same symbol.
Searching: by letter, by category, by name
There are three ways to find a symbol in the library. By name: "contactor", "thermal", "pump". By category: all the fuses in the "protection" category. By letter: the single-letter designations of IEC 81346. Q is a breaker, F is a fuse, K is a contactor, M is a motor, T is a transformer.
Searching by letter is a natural path for the person reading the drawing. A person who sees the tag "-K1" in a drawing and searches "K" in the library sees the whole class. In every situation where there is doubt about what a symbol is, a letter search reminds you of the designations.
What the number 802 covers
The 802 symbols are not a random pile. Contactors, relays, thermals, fuses, terminals, power symbols, PLC inputs/outputs, meters, transformers, earth and neutral symbols. The great majority of the symbols needed in a panel project are in this library.
The scope of the library shrinks the office's need to "draw a custom symbol". Custom symbols are left for devices that really are custom: a make-specific block, the office's own composite module. Drawing a custom symbol for a standard contactor is an unnecessary load.
Custom symbols: the library grows with you
Not every device will be in the built-in library. There can be a special device from a third maker, there can be the office's own composite block. In EupCAD you can import your own symbol into the library.
An imported symbol is not in a different status from the built-ins: it comes to the page with its pins, and with the addressing rules. The library is an asset of the office that grows as it draws; every custom device is not re-drawn in the next project, it is taken from the library.
One definition, many instances
A symbol on the page itself is a picture; what is real is the definition in the library. If the same contactor is used on thirty pages, that is not thirty copies of the same picture, it is thirty instances of the single definition in the library. This distinction decides the symbol's behaviour.
If something changes in the definition, say the position or the name of the pins, all the instances of that definition are updated. There is no need to correct thirty times by hand across thirty pages; the definition is updated once, and the instances follow it. As long as the symbol comes from the library, the instance and the definition do not break apart.
The addressing side of consistency
A symbol being drawn to the standard has only a visual benefit; the real benefit is on the addressing side. When you put a device on the page, the addressing behaviour comes with the symbol: the tag is laid out according to IEC 81346 rules, the terminals are labelled from that address.
So a symbol taken from the library carries two things: its appearance and its behaviour. A shape drawn by hand carries the appearance; it makes you rewrite the behaviour every time. As the library grows, the behaviour that has to be rewritten shrinks.
The newcomer draws his first page on his first day
An intern has just arrived and is going to add a page to the panel. In a classic office, he first asks "how do we draw a contactor here?"; the answer depends on the availability of the person who knows.
In EupCAD the intern takes the contactor from the library; it is the same contactor the senior engineer uses. His first page is not against the office style, because the office style is the library.
The library is the memory of the office
The most valuable asset of an office is the solutions it has gathered up. The custom devices from earlier projects, the common blocks, the tested circuit patterns, if these gather in the library, each new project does not start from zero.
An office without a library re-draws the previous project in every project. An office with a library continues from where the previous project left off.