SPST Relay

Single-pole single-throw relay with delays and faults

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Libraries:
Simscape / Electrical / Switches & Breakers / Relays

Description

The SPST Relay block models a single-pole single-throw relay. The block has three potential states:

• De-energized — The common contact, C, changes from being connected to disconnected from the normally open contact, S.

• Energized — The common contact, C, changes from being disconnected to connected to the normally open contact, S.

• Open Circuit — The relay is open. The common contact, C, is not connected to the normally open contact, S.

You can:

• Control the relay state using a physical signal input port or electrical conserving ports.

• Output the relay state.

• Delay the breaking and making of the connection.

• Introduce a behavioral or temporal fault that results in a stuck connection, an open circuit, or degraded contact resistance.

Relay State Control

The block has two control variants:

• Physical signal (PS) control — The state of the relay depends on how the value of the input physical signal compares to the relay threshold. The relay threshold, th, is the value that you by specify for the Threshold parameter.

• Electrical control — The state of the relay depends on how the current through the positive and negative electrical conserving ports, which represent the relay winding, compares to the relay upper and lower thresholds. The upper and lower thresholds depend on the values that you specify for the parameters in the Winding settings.

For the PS control variant, at the start of simulation:

• If the input signal, PS, is less than or equal to th, the relay is de-energized and the common contact, C, disconnects from the normally open contact, S.

• If the input signal, PS, greater than th, the relay is energized and C connects to the normally open contact, S.

After the simulation starts, if PS rises above th, the block goes from a de-energized state to an energized state and the C–S connection closes after the delay specified in Time-to-make C-S connection.

If the block goes from an energized state to a de-energized state, that is PS falls to or below th, the C–S connection breaks after the delay specified in Time-to-break C-S connection.

For the electrical control variant, the upper and lower thresholds depend on the specified values for the Rated voltage, Percent rated voltage to energize, Percent rated voltage to de-energize, and Winding series resistance parameters. The rated current is:

$$i_{rated}=\frac{V_{rated}}{r_{coil}}.$$

The equation for the upper current threshold, i_energized, is:

$$i_{energized\_current}=\frac{RatedVoltage*Percentratedvoltagetoenergize}{Windingseriesresistance}.$$

The equation for the lower current threshold, i_de-energized, is:

$$i_{de-energized\_current}=\frac{RatedVoltage*Percentratedvoltagetode-energize}{Windingseriesresistance}.$$

At the start of simulation, if the control current is greater than i_energized, the relay is energized and C connects to the normally open contact, S.

After the start of simulation, if the current rises above i_energized, the block goes from a de-energized state to an energized state. The C–S connection closes after the delay specified in Time-to-make C-S connection.

If the current falls below i_de-energized, the block goes from an energized state to a de-energized state. The C–S connection breaks after the delay specified in Time-to-break C-S connection.

Output the Relay State

To view the relay state, expose port x, a physical signal port that outputs the state of each connection. To expose the x port, in the Main settings, set State port to Visible.

The table shows how the state of the relay relates to the state of the connection. A closed connection has a state of 1. An open connection has a state of 0.

Relay and Connection States

C–S Connection State	Relay State
0	De-energized or open circuit
1	Energized

Connection Delays

You can specify delays for making and breaking connections in the Mechanical settings. The table shows how the make and break time delays affect the connections between contacts S1 and S2 and the common contact, C.

Mechanical Settings	Relay State	C–S Connection State	Results, with C–S in Blue and PS in Yellow
Parameter Value Time-to-break C-S connection 0 Time-to-make C-S connection 0	De-energized	0
	Energized	1
Parameter Value Time-to-break C-S connection > 0 Time-to-make C-S connection 0	De-energized	1, 0
	Energized	1
Parameter Value Time-to-break C-S connection 0 Time-to-make C-S connection > 0	De-energized	0
	Energized	0, 1

Faults

To model a fault in the SPST Relay block, in the Faults section, click Add fault next to the fault that you want to model. For more information about fault modeling, see Fault Behavior Modeling and Fault Triggering.

You can add a switch fault or an open-circuit fault to the winding. If you add a switch fault, you can set the Switch fault parameter to:

• C-S stuck closed

• C open circuit (no path to S)

• Degraded contact resistance

You can specify how and when faults occur by using the Trigger type parameter.

For more information about adding faults to blocks and specifying fault triggers, see Introduction to Simscape Faults.

C–S Stuck Closed Fault

The C–S stuck closed fault occurs if the common contact, C, becomes mechanically, electrically, or chemically stuck to the normally closed contact, S. Causes of this type of fault include:

• Contact welding, possibly due to an inrush current or breaking current that exceeds the rating or load short circuit.

• Mechanical damage.

• Insulation deterioration.

• Inductive voltage.

The connection state for a C–S stuck closed fault is equal to 1.

A C–S stuck closed behavioral fault can occur only if the load current exceeds the current threshold for a period of time that exceeds the behavioral trigger time threshold. That is, the behavioral fault can occur only if:

$$i_{load}>i_{th},$$

and

$$t_{i>i\text{\_}th}>t_{th\text{\_}b},$$

where:

• i_load is the load current.

• i_th is the specified value for the Maximum permissible load current parameter.

• t_{i>i_th} is the time that the current threshold is exceeded.

• t_{th_b} is the specified value for the Time to fail when exceeding current parameter.

When a fault triggers, if C–S is:

• Closed — C–S remains closed for the rest of the simulation.

• Open — The fault does not take effect unless the relay becomes energized and the C–S connection closes. Once closed, the C–S connection remains closed for the rest of the simulation.

C Open Circuit (No Path to S) Fault

The C open circuit (no path to S) fault occurs if the common contact, C, becomes mechanically stuck in an open position. This type of fault can happen if:

• There is contact damage or deterioration.

• The switch connector becomes mechanically stuck in the middle of the relay.

The faulted state for a C open circuit (no path to S) fault is equal to 0.

A C open circuit behavioral fault can occur only if the load current exceeds the current threshold for a period of time that exceeds the behavioral trigger time threshold. That is, the behavioral fault can occur only if:

$$i_{load}>i_{th},$$

and then

$$t_{i>i\text{\_}th}>t_{th\text{\_}b},$$

where:

• i_load is the load current.

• i_th is the specified value for the Maximum permissible load current parameter.

• t_{i>i_th} is the time that the current threshold is exceeded.

• t_{th_b} is the specified value for the Time to fail when exceeding current parameter.

When the temporal fault threshold is exceeded, if:

• The relay is in the open-circuit state, that is, the connection has been broken and not yet been made, the relay remains open for the rest of the simulation.

• The C–S connection is closed, the fault does not take effect unless the relay is de-energized and the closed connection is broken. If the connection is broken, the relay becomes an open-circuit, and remains open for the rest of the simulation.

When the behavioral fault thresholds are exceeded, if:

• The relay is in the open-circuit state, that is, the connection has been broken and not yet been made, the relay remains open for the rest of the simulation.

• The C–S connection is closed, the relay state immediately becomes an open circuit and remains an open circuit for the rest of the simulation.

Degraded Contact Resistance Fault

Causes of the degraded contact resistance fault include:

• Overuse-induced overload conditions. High inrush currents and voltages can cause overload conditions, as can excessive switching of the relay. Overload conditions ultimately trigger electrical arching, which generates heat that degrades the contact material.

• Chemical contamination that interferes with the operation of the relay contacts. Contaminants, which can include oxidation films or foreign particles, tend to produce high or unstable contact resistance readings.

• End of relay life.

The faulted state for a degraded contact resistance fault for the C–S connection is equal to either 0 or 1.

When a temporal fault first occurs, the contact resistance of S is:

$$r_{contact\_s}(t)=r_{contact\_fault\_s}-(r_{contact\_fault\_s}-r_{contact})sech\left(\frac{t-t_{th\text{\_}t}}{\tau }\right),$$

where:

• r_{contact_fault_s} is the final value of the faulted S contact resistance.

• r_contact is the unfaulted S contact resistance.

• t_{th_t} is the value of the Trigger fault at time parameter.

A degraded contact resistance behavioral fault can occur only if the load current exceeds the current threshold for a period of time that exceeds the behavioral trigger time threshold. That is, the behavioral fault can occur only if:

$$i_{load}>i_{th},$$

and then

$$t_{i>i\text{\_}th}>t_{th\text{\_}b},$$

where:

• i_load is the load current.

• i_th is the specified value for the Maximum permissible load current parameter.

• t_{i>i_th} is the time that the current threshold is exceeded.

• t_{th_b} is the specified value for the Time to fail when exceeding current parameter.

For a behavioral fault, if $$i_{S-C}>i_{th}$$ continuously over the time interval t_{th_b},

$$r_{contact\_s}(t)=r_{contact\_fault\_s}-(r_{contact\_fault\_s}-r_{contact})sech\left(\frac{t_{i>i\_th}-t_{th\_b}}{\tau }\right),$$

where:

• i_S-C is the common contact to normally closed contact, C–S, current.

• i_th is the specified value for the Maximum permissible load current parameter.

• t_{th_b} is the specified value for the Time to fail when exceeding current parameter.

• r_{contact_fault_s} is the final value of the faulted S contact resistance.

• r_contact is the unfaulted S contact resistance.

• τ is the value of the Time constant for degraded contact resistance parameter.

When the temporal fault threshold is exceeded for the C–S connection, the contact resistance is immediately degraded and remains degraded for the rest of the simulation.

When the behavioral fault thresholds are exceeded for C–S, in terms of i_S-C, the resistance for the C–S connection is immediately degraded and remains degraded for the rest of the simulation.

Winding failed open circuit

The open-circuit winding fault is available only for the electrical control variant. An open circuit in the winding coil can cause this type of fault.

For timed faults, the block approximates the winding current as:

$$L\frac{di}{dt}+Ri=v_{winding}*sech\left(\frac{t-t_{th\_t}}{\tau }\right),$$

where:

• L is the winding inductance.

• R is the winding resistance.

• i is the winding current.

• v_winding is the voltage across the winding.

• t_{th_t} is the value of the Trigger fault at time parameter.

• τ is the value of the Time constant for winding open circuit transition parameter.

A behavioral fault in the winding can occur only if one of these conditions is met:

• The winding current exceeds the current threshold for a period of time that exceeds the behavioral trigger time threshold.

• The winding voltage exceeds the voltage threshold for a number of times that exceeds the threshold for the number of voltage overloads.

That is, the behavioral fault can occur only if:

$$i_{winding}>i_{th}$$

and then

$$t_{i>i\text{\_}th}>t_{th\text{\_}b},$$

where:

• i_winding is the winding current.

• i_th is the specified value for the Maximum permissible winding current parameter.

• t_{i>i_th} is the time that the current threshold is exceeded.

• t_{th_b} is the specified value for the Time to fail when exceeding current parameter.

or if:

$$v_{winding}>v_{th},$$

and then

$$N_{v>v\text{\_}th}>N_{th},$$

where:

• v_winding is the winding voltage.

• v_th is the specified value for the Maximum permissible winding voltage parameter.

• N_{v>v_th} is the number of times that the voltage threshold is exceeded.

• N_th is the specified value for the Number of events to fail when exceeding voltage parameter.

Once a fault triggers, C–S remains closed for the rest of the simulation.

Limitations and Assumptions

• For behavioral faults, if Time to fail when exceeding current is greater than the time between switches, no fault is triggered because the accumulated heat is too low to melt or break the contacts or windings.

• The energize and de-energize delays can differ, but the energize delay must be greater than or equal to the de-energize delay.

Ports

The type, visibility, and location of the block ports depend on how you configure these parameters in the Main settings:

• Control port — Choose between a physical signal input port, PS, or electrical conserving ports, + and -, for relay control.

• State port — Set the visibility for the relay state physical signal output port, x.

Control port	State port	Block
PS	Hidden
	Visible
Electrical	Hidden
	Visible

Input

PS — Physical signal control

physical signal

Physical signal input port that energizes and de-energizes the relay.

Dependencies

To enable this port, in the Main settings, set the Control port parameter to PS.

Output

x — Relay state

physical signal

State of the relay in terms of the C-S connection:

• 1 — Closed connection

• 0 — Open connection

Dependencies

To enable this port, in the Main settings, set the State port parameter to Visible.

Conserving

S — Normally-closed contact

electrical

Electrical conserving port associated with the SPST relay normally-closed contact.

C — Common contact

electrical

Electrical conserving port associated with the SPST relay common contact.

+ — Winding positive terminal

electrical

Electrical conserving port associated with the SPST relay winding positive voltage terminal. The current through the winding controls the relay state.

Dependencies

To enable this port, set the Control port parameter, in the Main settings, to Electrical.

- — Winding negative terminal

electrical

Electrical conserving port associated with the SPST relay winding negative voltage terminal. The current through the winding controls the relay state.

Dependencies

To enable this port, set the Control port in the Main settings, to Electrical.

Parameters

Main

Control port — Input port choice

PS (default) | Electrical

Relay state control method, specified as:

• PS — PS physical signal input port

• Electrical — + and - electrical conserving ports associated with the relay winding terminals

The table shows how this parameter affects the visibility of other parameters in the Main settings. To learn how to read the table, see Parameter Dependencies

Control Port Parameter Dependencies

Control port
PS	Electrical
Closed resistance	Closed resistance
Open conductance	Open conductance
Threshold
State port	State port

Closed resistance — Connected contact resistance

0.01 Ohm (default) | positive scalar

Resistance across closed relay contacts.

Open conductance — Open-circuit conductance

1e-8 1/Ohm (default) | positive scalar

Conductance across open relay contacts.

Threshold — Relay threshold

0 (default) | nonnegative scalar

Threshold of the relay. If the physical signal input rises above the threshold value, the relay is energized. Conversely, if the physical signal input falls below the threshold value, the relay is de-energized.

Dependencies

To enable this parameter, set Control port to PS.

State port — Relay state output port visibility

Hidden (default) | Visible

Visibility of the physical signal port that outputs the relay state. This parameter value is 1 if the connection is closed, and 0 otherwise.

To enable the x port, set this parameter to Visible.

Winding

To enable these parameters, in the Main settings, set the Control port parameter to Electrical.

Rated voltage — Rated voltage

5 V (default) | positive scalar

Standard voltage applied to the operating coil under normal operating conditions. The current threshold depends on the value of this parameter.

Percent rated voltage to energize — Percent rated voltage to energize

75 (default) | positive scalar

Minimum percent of the rated voltage required to energize the relay. The current threshold depends on the value of this parameter.

Percent rated voltage to de-energize — Percent rated voltage to de-energize

25 (default) | positive scalar

Percent of the rated voltage required to de-energize the relay. The current threshold depends on the value of this parameter.

Winding inductance — Winding inductance

0.08 H (default) | positive scalar

Winding inductance.

Winding series resistance — Winding series resistance

12.5 Ohm (default) | positive scalar

Resistance between the winding terminals. The current threshold depends on the value of this parameter.

Winding parallel conductance — Winding parallel conductance

1e-9 1/Ohm (default) | positive scalar

Winding conductance.

Mechanical

Time-to-break C-S connection — Delay time for breaking connection C-S

0 s (default) | nonegative scalar

Mechanical switching time for breaking the connection between ports C and S when the relay is energized.

Time-to-make C-S connection — Delay time for making connection C-S

0 s (default) | nonegative scalar

Mechanical switching time for making the connection between ports C and S when the relay is de-energized.

Faults

Switch fault — Option to add switch fault

Add fault

Option to add a switch fault.

To add a fault, click the Add fault hyperlink.

Switch fault — Switch fault model

C-S stuck closed (default) | C open circuit (no path to S) | Degraded contact resistance

Switch fault model.

Dependencies

To enable this parameter, click the Add fault hyperlink for the Switch fault parameter.

Trigger type — Trigger type

Always on (default) | Timed | Behavioral | Additional triggers | Conditional | Manual

Trigger type, specified as one of these options:

• Always On — The fault injects at the start of the simulation.

• Timed — The fault injects when the simulation time reaches the value you specify for the Trigger fault at time parameter.

• Behavioral — The fault injects as a result of fault parameter values during simulation. Setting Trigger type to Behavioral enables block parameters that define the failure conditions.

• Additional triggers — The fault injects as a result of additional triggers not available in the block dialog box. Select this option and click the Open fault properties hyperlink to set the Trigger type to one of these values in the Property Inspector:

  • Conditional — The fault injects as a result of a condition that reflects a behavior associated with a signal. Conditionals evaluate the Boolean expression in the Condition parameter at each time step. The block enters the faulted state when the trigger condition becomes true for the first time and remains in the faulted state for the rest of the simulation. To learn more, see Create and Manage Conditionals.

  • Manual — The fault injects or clears when you toggle the status in the Fault Dashboard pane. To learn more, see Manually Trigger Faults in Models.

Dependencies

To enable this parameter, click the Add fault hyperlink for the Switch fault parameter.

Trigger fault at time — Time before entering faulted state

0 s (default) | nonnegative scalar

Simulation time at which the block enters the faulted state.

Dependencies

To enable this parameter, click the Add fault hyperlink for the Switch fault parameter and set Trigger type to Timed.

Maximum load permissible current — Load current threshold for fault transition

inf A (default) | positive scalar

Current threshold for a fault transition. The block enters the faulted state if the current continuously exceeds this value for a period longer than the value of the Time to fail when exceeding current parameter.

Dependencies

To enable this parameter, click the Add fault hyperlink for the Switch fault parameter and set Trigger type to Behavioral.

Time to fail when exceeding current — Time to fail when exceeding current

1 s (default) | positive scalar

Amount of time that the current must continuously exceed the maximum permissible current before a fault triggers.

Dependencies

To enable this parameter, click the Add fault hyperlink for the Switch fault parameter and set Trigger type to Behavioral.

Trigger configuration — Option to enable additional trigger types

Open fault properties

Option to enable additional trigger types not available in the block dialog box. To set the Trigger type parameter to Conditional or Manual, click the Open fault properties hyperlink and set the Trigger type in the Property Inspector.

Dependencies

To enable this parameter, click the Add fault hyperlink for the Switch fault parameter and set Trigger type to Additional triggers.

Degraded contact resistance — Degraded contact resistance

1e-9 Ohm (default) | positive scalar

Degraded resistance of the faulted S contact.

Dependencies

To enable this parameter, click the Add fault hyperlink for the Switch fault parameter and set Switch fault to Degraded contact resistance.

Time constant for degraded contact resistance — Time constant for degraded contact resistance

1e-3 (default) | positive scalar

Time constant for contact resistance to fall into degraded status.

Dependencies

To enable this parameter, click the Add fault hyperlink for the Switch fault parameter and set Switch fault to Degraded contact resistance.

Winding fault — Option to add winding fault

Add fault

Option to add a winding fault.

To add a fault, click the Add fault hyperlink.

Dependencies

To enable this parameter, set Control port to Electrical.

Time constant for winding open circuit transition — Time constant for winding to fail

1e-3 s (default) | positive scalar

Time constant for the winding to fail. This constant is the characteristic time that describes a transition from a normal state to an open circuit. You can calculate this value as the time it takes for the response to reach 63% of its steady-state value when the fault occurs.

Dependencies

To enable this parameter, set Control port to Electricaland click the Add fault hyperlink for the Winding fault parameter.

Trigger type — Trigger type

Always on (default) | Timed | Behavioral | Additional triggers | Conditional | Manual

Trigger type, specified as one of these options:

• Always On — The fault injects at the start of the simulation.

• Timed — The fault injects when the simulation time reaches the value you specify for the Trigger fault at time parameter.

• Behavioral — The fault injects as a result of fault parameter values during simulation. Setting Trigger type to Behavioral enables block parameters that define the failure conditions.

• Additional triggers — The fault injects as a result of additional triggers not available in the block dialog box. Select this option and click the Open fault properties hyperlink to set the Trigger type to one of these values in the Property Inspector:

  • Conditional — The fault injects as a result of a condition that reflects a behavior associated with a signal. Conditionals evaluate the Boolean expression in the Condition parameter at each time step. The block enters the faulted state when the trigger condition becomes true for the first time and remains in the faulted state for the rest of the simulation. To learn more, see Create and Manage Conditionals.

  • Manual — The fault injects or clears when you toggle the status in the Fault Dashboard pane. To learn more, see Manually Trigger Faults in Models.

Dependencies

To enable this parameter, click the Add fault hyperlink for the Winding fault parameter.

Trigger fault at time — Time before entering faulted state

0 s (default) | nonnegative scalar

Simulation time at which the block enters the faulted state.

Dependencies

To enable this parameter:

• Set Control port to Electrical.

• Click the Add fault hyperlink for the Winding fault parameter.

• Set Trigger type to Timed.

Maximum permissible winding voltage — Maximum permissible winding voltage

inf V (default) | positive scalar

Maximum voltage for the relay winding. The block enters the faulted state if the voltage exceeds this value for the k^th time, where k is the value of the Number of events to fail when exceeding voltage parameter.

Dependencies

To enable this parameter:

• Set Control port to Electrical.

• Click the Add fault hyperlink for the Winding fault parameter.

• Set Trigger type to Behavioral.

Number of events to fail when exceeding voltage — Number of events to fail when exceeding voltage

1 (default) | positive scalar integer

Number of times the relay must exceed the winding voltage threshold before a open-circuit winding fault triggers.

Dependencies

To enable this parameter:

• Set Control port to Electrical.

• Click the Add fault hyperlink for the Winding fault parameter.

• Set Trigger type to Behavioral.

Maximum permissible winding current — Maximum permissible winding current

inf A (default) | positive scalar

Current threshold for a fault transition. The block enters the faulted state if the current continuously exceeds this value for a period longer than the value of the Time to fail when exceeding winding current parameter.

Dependencies

To enable this parameter:

• Set Control port to Electrical.

• Click the Add fault hyperlink for the Winding fault parameter.

• Set Trigger type to Behavioral.

Time to fail when exceeding winding current — Time to fail when exceeding winding current

1 s (default) | positive scalar

Amount of time that the current must continuously exceed the maximum permissible current before a fault triggers.

Dependencies

To enable this parameter:

• Set Control port to Electrical.

• Click the Add fault hyperlink for the Winding fault parameter.

• Set Trigger type to Behavioral.

Trigger configuration — Option to enable additional trigger types

Open fault properties

Option to enable additional trigger types not available in the block dialog box. To set the Trigger type parameter to Conditional or Manual, click the Open fault properties hyperlink and set the Trigger type in the Property Inspector.

Dependencies

To enable this parameter:

• Set Control port to Electrical.

• Click the Add fault hyperlink for the Winding fault parameter.

• Set Trigger type to Additional triggers.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2019b

R2024a: Implement faults using the Simscape faults interface

The SPST Relay block now supports the Simscape faults interface. To learn more, see Introduction to Simscape Faults.

As a result of this change, the parameter names and dependencies in the Faults settings have changed.

R2023a: Implement temporal or behavioral triggers for faults using a single parameter

The Enable temporal trigger and Enable behavioral trigger parameters have been removed and are replaced with the Fault trigger parameter. You can set the Fault trigger parameter to Temporal or Behavioral. You can enable only one trigger.

If you created a model in an earlier release and enabled both temporal and behavioral triggers, now only the temporal trigger is enabled. Therefore, the simulation results are different if the behavioral trigger causes the fault in your model.

If you want to trigger a fault at a specific time, set the Fault trigger parameter to Temporal. If you want to determine whether a system fails and, if so, when it fails, set the Fault Trigger parameter to Behavioral.