# How to understand schematic circuit diagrams in functional safety

I'm having a hard time understanding the kind of diagrams which frequently appear in literature relating to ISO 13849.

Are there relevant instructions or guides on how to understand this kind of diagrams?

1. What do the dashed lines mean?
2. Why are there three different symbols with the Q1 label? Do the three different symbols have different meanings?
3. Why is there a part (which I marked with a green rectangle) separate from others?
4. How can I derive from Figure 2 and the context that the two channels are B1-Q2 and B2-K1-Q1 as stated in Figure 3?

1. What do the dashed lines mean?

In the case of Figure 2, Q1 they show the mechanical connection between the coil and the contact. This is a bit redundant as both are labelled Q1.

In the case of the Figure 2 cam switches B1 and B2 the contacts aren't named and so the dotted line indicates which contacts are actuated by the cam-following rollers. In the position shown, B1 not actuated, the left contact is closed and the right contact is open.

1. Why are there three different symbols with Q1 label? Do the three different symbols have different meanings?

Q1 is a relay which by definition will have a coil and at least one contact. The symbol on the bottom right is the coil, the bottom left is a normally-open contact and the upper one is a normally-closed contact.

1. Why is there a part (which I marked with a green rectangle) separate from others?

The three diagonal lines indicate that this is a three-phase power circuit for a three-phase motor. The relays are used to control (with low voltage / low current) a high power load. The relay isolates the high voltage circuit from the low voltage control circuit.

1. How can I derive from Figure 2 and the context that the two channels are B1-Q2 and B2-K1-Q1 as stated in Figure 3?

I don't think you can. The PLC is a programmable device and we can't see what the program is.

I don't think that this is good material. It gives the impression that a standard PLC can be used as a safety control system. This is generally not true as the failure mode is not predictable. (The output transistor could fail as a short-circuit or open-circuit.)

You might find my answer to this post useful: https://electronics.stackexchange.com/a/240352/73158

The dashed lines represent physical interconnects, either microswitch actuator to contact (B1 and B2),or relay coil armature to associated NO and NC contacts (Q1 and Q2).

The "separate" circuit in green is the relay contacts that form the safety interlock on the motor drive.

The contexts in the second diagram show the elements which control each of the interlock contacts for the motor. As noted in the text, the thick lines are drawn backwards from the motor contacts.

What you can see is the multiple levels of redundant protection, all of which have to operate correctly for the motor to be enabled.

These kind of standards like to make simple things as complicated as possible... even though it is well established that increased complexity leads to more errors. What you need for a "reduntant" safety circuit is just 2 pcs of 2 pole NO safety relays with forcibly guided contacts.

One contact of each relay connects the actual signal. These are commonly placed in series with each other. The other contact of each relay is connected to a known voltage, which is routed to the MCU/PLC when the relay is activated. The MCU/PLC also drives the coils. So it can verify if the relays actually ended up in the state it expected.

Conceptually the circuit might look something like this, where U1 is the MCU/PLC with 2 outputs and 2 inputs:

simulate this circuit – Schematic created using CircuitLab

24VDC is the common industry standard most often used, hence the need for voltage dividers. Depending on the quality of the supply, additional protection of the MCU/PLC might be needed too.

From an ISO 13849-1 perspective, the above is a "category 2" implementation, so you can get somewhere around "pl c"/"pl d". For increased safety levels ("category 3/4"), two MCUs are used as well, one for driving each relay, both of them supervising both relays and each other.