Documentation & Drawing Standards
Disclaimer
The opinions below are my own rantings and are probably
missing important information and caveats.
Don't rely on it alone. Some reference are listed below:
The PC Schematic book
https://cdn.pcschematic.com/pdf/en/standards-book/ElectricalDocStdV3_p1-30.pdf
The 81346 Handbook
https://webshop.ds.dk/standard/M337006/ds-handbook-166-2020-e
The 81346 Website
https://www.81346.com/
Background
In Australia, we don't have a current, widely recognised
drawing standard. There are several common approaches such as
using the IEC style, using the ANSI style, or rolling their
own. The roll your own approach tends to mix the horizontal
rungs of the ANSI system, but using the IEC symbols. Letter
codes are often based on a combination of the superseded AS
3702 letter codes, ISA5.1, ANSI Y32.3, and whatever that
person felt like at the time.
Why you should not roll your own
Rolling your own system causes many problems. They are listed
below.
IEC/ISO 81346 resolves a lot of them. You never run out of
tags with 81346. You can create tags independently without the
risk of creating duplicates. It gives you extensive letter
codes, while they are arbitrary, are at least standardised.
Why Fixed Length Tags don't work
Fixed length tags inevitably fail. This is because it is
almost always possible to have another component associated
with the main component. For example, you might try to make a
motor tag be 100FT01M01. This might mean Process line 1,
Filter 1, Motor 1. But what happens if you need to
install a motor that has a cooling fan motor? Does it become
M02? What if M02 is already assigned to something else?
Maybe M22?
IEC/ISO 81346 has the solution. The tags are not fixed length.
If you need to add a motor to a motor, the motor just becomes
M1.M1. This means that you never run out of tags, and
you can unambiguously designate anything to any level of
detail. Notice the word unambiguous versus unique. The
markings are not unique. You may find dozens of M1's on a
site. The reason these don't get mixed up is that the upstream
part of the reference designation is documented elsewhere, so
there is no confusion about which M1 is which. If it is
not obvious, then the relevant upstream part of the tag is
added, for example +L1-M1, +L2-M1 for the motors M1 in
locations 1 and 2.
Why Numbers should not mean anything
The reason for this is mathematical. There are only 10
digits. It is unlikely that a system of any complexity can be
neatly split into 10 categories. You might try to make more
categories by using more digits, 100 for one category, 150 for
another, 200, but this reduces how many items you can put in
each category. What if there are 51 devices in the 100
category? If you put the 51st devices as 1105, you've just
created a duplicate category. If you put it in the 150
category, that category now contains devices of different
types or functions. Repeat this a few times and there
are so many exceptions to the the numbering rules that you end
up with the situation that a number doesn't mean anything
anyway. Engineering installations last a long time and
will undergo significant changes during their lifespan. If you
could reliably predict what these changes would be, you would
not be here reading about drawing numbers. What I'm saying is
that you can never accurately reserve categories for future
works. The rule for numbers is that they are to
differentiate the same type of item. M5 just means that it's
the motor designated M5. It doesn't even mean that it is the
5th motor, since design changes could have removed a motor.
Why you should not roll your own letter codes
Rolling your own letter codes might seem easy at first. CB for
circuit breaker, V for valve, M for motor, PU for pump. Then it
gets harder. How about a compressor? CP? How about a control
panel? CP as well? Then it gets worse. Network switch vs router?
Modem? In a larger organisation this can cause a lot of
problems. Who gets to decide the letter codes? Who keeps track
of which codes are used for what? What happens if there is
disagreement about which codes to use? Who resolves it?As always, the answer is IEC/ISO 81346. While the letter codes are arbitrary, they are standardised and recognised. Once the decision has been made to use the standard, there are only going to be smaller decisions that need to be made about how many letters to use, and which items should be assigned to which letter codes. The standard provides useful guidance for which letter code to use for a large number of device types.
Why you should not tie drawing numbers to reference
designations (tags)
I have seen drawings that use the sheet and position of the
symbol for the reference designation. The idea is that you can
look at a tag, for example 10F10, and know that it appears on
column 10 of sheet 10.
While it might seem like a neat idea, it causes terrible
problems on projects that need to be designed fast, changed
during the design or build, or have more than one panel. For
modern engineering, changes during the design are inevitable.
Workflow is slower. You have to finalise the schematics before
you can create a layout. If there are changes, then you can
create inconsistencies. If you need to add one circuit breaker
at the end of a row, and you've put them on sheet 10, the last
one might be 10F10. If you don't have space on sheet 10, then
the new one might be sheet 31F1. Do you then move everything
that is already drawn and labelled? Or do you live with this
weird number?
I have also seen wire numbers use this approach. The idea is
that a wire might appear on sheet 10, column 01, and be the
11th wire. 100111.
It means that you have to complete the schematic before you
can assign the wire numbers.
In my opinion, valid approaches are to use source/destination
designations or signal designations. For example, a wire that
is connected to F1, terminal 2 on one end and X1.1 on the
other, would have F1-2/X1.1 on both ends. If it is a doubled
connection, with one wire coming from F1-2, one wire coming
from X2.1, both being connected to X1.1, you put all three
designations F1-2/X1.1/X2.1.
Signal based numbering is also a good option, but you should
take guidance from IEC 61175-1.
Why the "fixing it at 3am" argument is misleading
The argument for sheet based typically invokes a hypothetical
electrician who is tasked with fixing something urgently. The
scenario put forward is that they can see the contactor, 10Q10
not activating, so they can go to sheet 10, column 10 and see
what is causing that contactor to not pull in. On the surface,
it sounds convincing. Yes, it might take more time to generate
a tag like 10Q10, but it saves on downtime in the
future. If the drawings are lost, tag and wire number
mean nothing. The electrician will look at the wire number to
try and trace it. If the wire number is something
source/destination, they can follow the wire without having to
take it out of the duct or search the entire panel for the
same number. If the drawings are present, ideally, there
will be a sheet that lists which device is on which sheet. In
purpose-built electrical CAD program such as Eplan, it is
fairly straightforward to make a table that lists what sheets
each device is listed on. Even if that list isn't present, if
the drawings are structured, there won't be too many sheets to
look through. The electrician knows what panel they are
looking at, so while the drawing set may have 1000 sheets, the
panel itself might only have a dozen or so drawings relevant
to the section with the contactor that is not actuating.
Why you should not try to make your drawing numbers mean
anything
This is a similar issue to that of fixed length tags. If you
try to make your drawing something like this 2001-DWG-200,
where 2001 might be a project number, DWG means drawing, and
200 is the sheet number, and the sheet number means something
(200 for power distribution or something), you are going to be
in a world of hurt as soon as you are building a project with
more than one panel. You might think that this is manageable.
It is not. I have seen people valiantly try this approach with
projects where the number of drawings gets perilously close to
1000. They carefully allocate numbers to each panel. It falls
over fast. The customer needs a new panel. A large panel gets
split into two panels. A small panel gets an additional IO
rack. Suddenly, a panel that was supposed to have drawings
from 200 to 250 needs another 50 pages. Do you allocate an
unused number? Do you prefix the number with a 1, for example
1251 for the page after 250?
The problems are even worse if you tie the reference
designations (tags) to the sheet. If you have an identical
panel, you cannot copy the drawings, every tag, every wire
number, every cross reference needs to be changed. This flows
further through the documents, cable termination diagrams
cannot be copied, test procedures cannot be copied, labels
cannot be copied etc.
The solution is to structure your project using IEC/ISO 81346
principles and just use a sheet number that doesn't mean
anything specific. The structure information should be
on the drawing and the drawing file structured accordingly.
Also, do not tie reference designations (tags) to the sheet.
This allows you to treat documentation as interchangeable and
re-usable modules. For a duplicate control panel, all you need
to do is changes the appropriate part of the designation, and
everything else can be the same. For site maintenance and
usability this can be very helpful. The main switch might
always be Q1 on the panel, rather than panel 1 being 10Q04,
panel 2 being 20Q04, and the last minute addition panel being
110Q04 etc.
Why IEC/ISO 81346 is not used widely in
Australia
In my opinion, the reason why IEC/ISO 81346 is not widely
used in Australia is that we mostly have small installations,
and the problems that we experience with reference
designations are low on the priority list compared to the
other problems. The existing systems, while they do
cause problems, work well enough to keep things going. There
is also the knowledge gap. People in charge of installations
do not typically understand what they are in charge of, and
why they even need a reference designation system at all. The
costs of not following the standard show up as increased
engineering hours, and are not easily attributed to reference
designation systems.
So what should I do?
My advice is this:
- Use a real electrical CAD program. Autocad does not count.
It does not provide automated page management, cross
references, linking of symbols and layouts, methods of
generating tables, etc. I will write a page in the
future for those of you unfortunate enough to be forced to
use a program like this. My preference is for a program
written to be an electrical CAD program, rather than an add
on for an existing program like AutoCAD Electrical, Elecdes,
or Radica. The reason for this is my personal experience. It
is too easy to inadvertently use the non-smart method to do
something that breaks the functionality built on top of the
base program, causing your drawing package to revert back to
a dumb system.
- Structure your project with the IEC/ISO 81346 principles.
Structure it so that duplicate machines/areas/control panels
are not in the same structure identifier. This allows you to
copy and paste duplicate drawings. Always create a map to
show where the structure identifiers are located (where
applicable)
- Keep your reference designations "dumb". Don't try to make numbers don't mean anything, use the letter codes from IEC/ISO 81346
- Structure your drawing package, but don't tie it to the page number. Job number - Sheet number is fine. Put the structure as part of the title block
- Break up your PLC cards. Focus your sheet on a device. For example, for a valve with two solenoids and two limit switches, focus the drawing on the valve. Show each individual channel applicable to the valve on the sheet. This makes field wiring and diagnosis easier, as you won't be flipping between sheets.
- Show a PLC overview per card per sheet. This allows you to see what each channel on a PLC card goes to.
- Know which devices to break across sheets and which ones not to break. Field devices should not be broken up unless it is not possible to fit them on a single sheet. Devices that are in a panel such as PLC cards, multi channel IS barriers, terminals, these should be broken up to avoid breaking up field devices. This reduces a common complaint about IEC style drawings, that the signal you're trying to trace is split across too many sheets.
- Show items that are external to the enclosure in a defined
structure box. Don't make the structure box "Field", define
it in such a way that it is useful. For example, if you have
a gate switch in the press section of the machine, define a
press section +PR1 in the document structure. On the
schematic, show that the gate switch is located within +PR1.
The same approach could be used for a functional aspect. For
example, if a pressure switch is in the field as part of the
water filtration system, you could designate the water
filtration system as =H1 and show that the pressure switch
is in =H1
What a drawing set should include
- Title Sheet
- Table of Contents - shows the revisions of each drawing
- Instruction Sheet that explains conventions used within
the drawing set. For example, if there is no colour or size
shown, then you expect a white wire, 0.5mm cross section. Or
if a cable is specified without a conductor material, it is
copper.
- List of Structure Identifiers (avoid capital I and capital
O to prevent confusion with 1 and 0)
- Device Tag List - This should list the part number for
each device, and which sheet the main function (such as the
coil) appears on
- Structure Identifier Map. This shows a simplified layout
of the plant, but has the structure identifiers
superimposed, so that the installers know approximately
where to find the field devices.
- PLC Rack Layout
- PLC overview sheets (one or two cards per sheet, showing a
summary of what each channel is assigned to and what page to
refer to for further detail)
- Schematics that are focused per device