The light switch in your room is a 2-wire system. A single switch turns on and turns off the light. Two wires are required at the light.
I tried to think of a household example of a 3-wire system in household use and wasn’t able to think of a commonly used example. Most circuits in a home use 2-wire systems. An example from industry is a fan or pump which is started with as Start button and stopped with a Stop button. This requires a minimum of 3 wires to the pump starter.
It is clear the 2-wire system is less expensive.
One consideration with 2-wire motor starters is that many safety codes require that if power is lost and restored, the machine shall not start without the operator re-initiating the start. No harm with a light switch, but there is with a table saw, or in-running nip, or a slitter blade.
A 3-wire system will automatically stop and remain stopped after a power outage.
Are either of these systems fail-safe? On a 2-wire system, if any wire is cut, the motor will stop. On a 3-wire system if the stop wire is cut, the motor will stop. If the start wire is cut, the drive will continue to run, but cannot be re-started. Both of these are fail-safe.
We do not need the buttons. A PLC or drive controller can start or stop drives with hardwired outputs using a 2-wire or 3-wire system. Safety comments are identical to the button case.
We do not need the hardwired signals. The PLC or drive controller can start or stop the motor over a communication link using a 2-wire or 3-wire system. These systems can be easily programmed to prevent restart on loss and restoration of power. Separate means such as stop on communications fault are required to make this system fail-safe.
In all cases the 2 wire system is lower cost. Safety considerations are equivalent for either method.
Note: a risk assessment may determine that more than 2 and more than 3 wires are required to achieve the required safety performance level or classification. The discussion above is for normal starts and stops, not safety interlocked stops.
Thursday, November 14, 2013 6:14 PM
Would a deadman's switch on a lawnmower be a 3-wire system? It won't start without the deadman's switch engaged and will stop if either 'off' or the swigch are triggered.
Friday, November 15, 2013 1:27 AM
The deadman's switch as wired in the past could have been done with either 2-wire or 3-wire methods. A contact on the switch would have interrupted power to the run command or initiated a stop command.
Today, if a deadman's switch was required, the risk assessment would probably call for a certain performance level (PL) or CATegory or SIL level of the system. The level would most likely require two (redundant) 2-wire systems for the deadman's switch, several wires to connect to the source of power (lawn mower motor), and two more wires to verify that the motor has stopped.
A lot has happened in industrial safety in the past 20 years and requirements are increasing all the time. Presently the EU Performance Level (PLa to e) classifications most completely define safety performance over the life of a machine and for all the tasks anticipated for that machine.
Tuesday, October 20, 2015 5:21 PM
I have to completely disagree with the statement: "In all cases the 2 wire system is lower cost. Safety considerations are equivalent for either method."
You are forgetting about the Processing/Signal time for a PLC to drop its output on loss of control power. In most cases there is still a hardwired Stop button as well, and if this is installed as a Momentary on a 2 wire control from a PLC/DCS you have a huge safety concern, and equipment failure concern.
Tuesday, October 20, 2015 11:18 PM
You are correct about the scan time using a PLC. The PLC adds its scan time plus 2 I/O scans plus communication time to the drive. This time may or may not be significant compared with the drive stopping time.
A normally open pushbutton should never be used with a two wire start circuit.
A risk assessment with determine the appropriate safety performance. A an approved latching E-Stop and guard interlocks may be needed. The buttons will have to be arranged in a recognized safety architecture (category). This may involve safety relays, safety PLCs, Safe Torque Off (STO) or Safe Speed drives.
I retract my statements about safety in regard to the start circuit. A risk assessment will be required before designing the circuit.