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Relay Basics

General Knowledge of Relays

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General Application

Rating

Relay ratings include the coil ratings and contact current ratings.

1. Coil Specification

For actual use, be sure to not exceed the coil rating; it can lead not only to performance loss but also to burn out the coil caused by overvoltage etc. Be sure to carefully select the AC coil specification by checking the applicable power source of each relay (rated voltage, rated frequency).

Certain types of relays may not tolerate under specific rated voltage and rated frequency.
If used under such condition, it can cause abnormal heating and malfunction.
The following chart shows the AC coil specifications.

Example: 100 VAC
Rating names * Applicable Power Source
(Rated Voltage, Rated Frequency)
Product Labels Catalog description
Rating 1 AC 100V 60Hz 100VAC 60Hz AC 100V 60Hz
Rating 2 AC 100V 50Hz, AC 100V 60Hz 100VAC AC 100V
Rating 3 AC 100V 50Hz, AC 100V 60Hz
AC 110V 60Hz
100 / 110 VAC, 60 Hz 100 VAC,
50Hz or 100 / (110) VAC
AC 100 / (110) V
Rating 4 AC 100V 50Hz, AC 100V 60Hz
AC 110V 50Hz, AC 110V 60Hz
100 / 110VAC AC 100 / 110V

*Note: that the rating names indicated here are not officially specified by Japanese Industrial Standards (JIS) or the like.

2. Contact Specification

Contact ratings are the standard values for guaranteed relay performance and generally indicates the current rating of the relay contacts.
The rating varies depending on the voltage applied and the types of electrical loads. In other words, the rating includes the specification of the maximum voltage applied to the relay contacts and the maximum current that can be passed to control the electrical load.

Contact Specification
  • Contact ratings are generally indicated according to resistive loads.
    Be sure to choose the right kind of relay applicable to the electrical load you are controlling and that meets your durability requirements.

Inrush Current

Inrush current is the large current that flows instantaneously when the power is first turned on and supplied to the electrical circuit to control the load, exceeding the steady state current value.
This occurs with electrical loads such as electric motors and incandescent light bulbs.

1. Inrush Current
  • Resistive load
    Resistive load

    The current remains at constant level immediately after the power is turned on.

  • Lamp load
    Lamp load

    Inrush current of approximately 10 times larger than the steady state current flows immediately after the power is turned on, then returns to its constant level.

2. Inrush Current and Ratings

The TV rating is one of the representative ratings approved by UL and CSA regulations to evaluate the inrush current withstand capability. The rating indicates the level of relay’s capability to switch the load, including the inrush current.

For example, relays for television power supplies need to obtain the TV rating.
TThe switching test (durability test) of these relays is performed using a tungsten lamp as a load and must withstand in total 25,000 times of the durability test.

TV rating Inrush current Steady state current Example of product types
TV-3 51 A 3 A G2R-1A
G2RL-1A-E-ASI
TV-5 78 A 5 A G5RL-1A(-E)-LN
TV-8 117 A 8 A G4W-1112P-US-TV8
G5RL-U1A-E
G5RL-K1A-E
G5RL-1A-E-TV8
TV-10 141 A 10 A G7L
TV-15 191 A 15 A G4A

DC Circuits

Arc is an electric spark that occurs between the contacts when the relay closes the electric circuit.
As the voltage and current amplitude increase, the arc rises. When the switch is closed slowly, it takes longer time for an arc to form. This can cause the contacts to wear out quickly.

Switching DC circuits
Switching DC circuits

In alternating current (AC), which constantly changes its direction of flow, the arc is quenched every time an overvoltage is delivered.
On the other hand, indirect current (DC) only flows in one direction, which allows forming an arc to take longer, leading to quicker contact wear out and durability decrease.

Also, a transition phenomenon of the contact occurs, which can cause irregularities at the contact points, which can cause malfunctions that can not be separated because they are caught.

  • Contacts that are connected in series will increase the contact gap in equal length, allowing effective control of the arc.

Minimum Load Application

A relay may face a problem of contact resistance build-up when switching minimum load applications. Whenever there is a rise in contact resistance, contacts would normally recover by the subsequent operation. Contact resistance may also increase, caused by film formation.

To determine if the measured contact resistance value predicts a relay failure shall depend on whether it is causing a circuit problem or not.
For this reason, only default values are specified as standard failure rates of relay contact resistance. Failure rates (*) are expressed as P level (reference value) as one indicator for minimum applicable loads.

*Notes: Failure rates

The percentage of failures per unit time (or number of operations) during continuous relay switching under individually specified test types and loads.

The rate may vary, depending on the switching frequency, ambient conditions and expected reliability level. Therefore, users must test the relay under actual operating condition to verify its applicability.

In this catalog, the failure rate is given as the P level (reference value). This expresses the failure level at a reliability level of 60% (λ 60) (JIS C5003).

Lamp load
Using Relays with a Minimum Load Application

When selecting a suitable relay to switch a minimum load application, be sure to consider the type of load you are switching as well as the required contact material and the contact arrangement.

The contact reliability when controlling minute loads greatly depends on the contact material and contact arrangement.
For example, twin contact points are more reliable than single contact point for minimum load applications simply from the reason that redundancy in parallel operation of twin contact provides greater reliability than is offered by single contact.

Durability

Durability (life) of a relay is the number of times the relay can switch until it fails to meet the specified values in terms of operating characteristic and performance.
Relay durability is divided into two categories: Mechanical Durability (relay life) and Electrical Durability (relay life).

Mechanical durability (relay life)
This is to see how many cycles the relay can operate at the specified switching frequency with no load applied to the contacts.
Electrical durability (relay life)
This is to see how many cycles the relay can operate at the specified switching frequency with the rated load applied to the contacts.
Switching Capacity

Users shall check the maximum switching capacity of each relay, using graphs to find a relay suitable to their applications.
Maximum switching capacity and durability curve can be used as guidelines for selecting a relay.
Note that the values obtained here are only guidelines; the relay must be tested under actual load condition.
The following shows how to read the graphs of maximum switching capacity and durability curve.

For example, if contact voltage (V1) is already determined, the maximum contact current (I1) can be obtained from the intersection point on the characteristic curve.
Conversely, if the maximum contact current I1 is already determined, contact voltage (V1) can be obtained.
Then the I1 obtained is used to obtain the number of operating cycles from the durability curve.

Example on these graphs:
If contact voltage is 40V,
Contact switching current is up to 2A …… *1
The number of operating cycle at maximum contact current 2A is approx. 340,000 times …… *2

  • Maximum switching capacity
  • Durability curve
  • Relay durability varies greatly with the types of loads, switching conditions and ambient conditions; relay operation must be tested and evaluated under actual conditions.

Failure Analysis

Users may face certain problems associated with relays when operating their equipment.
In such cases, the cause needs to be identified using the FTA (Faulty Tress Analysis) method.
The following chart lists specific failure modes and the possible causes.

Problems Visible from Outside the Relay
Failure events Checklist Possible causes
Relay is not working 1. Voltage may not be properly applied to the relay input
  • Fuse blown or breaker tripped
  • Incorrect wiring, possible leaking
  • Loose terminal screw connections
2. Relay specification may not be properly selected for the input voltage being used with it.
  • An AC voltage of 200 V was applied to the relay with rated voltage of 100 VAC.
3. There may be input voltage drops.
  • Insufficient power supply capacity
  • Long wiring distance
4. The relay may be damaged.
  • Disconnection of the relay coil
  • Caused by dropping, damage by mechanical shock
5. The output circuit may not be working properly.
  • Check the power source on the output side
  • Load failure
  • Incorrect wiring
  • Connection failure
6. The relay contacts may not be working properly.
  • Poor contact alignment
  • Worn out contacts (enclose to end of relay life)
  • Mechanical failure
No signs of recovery of the relay 1. The voltage may not be applied to the relay at all.
  • Current leaking from the protection circuit (surge absorber)
  • Voltage applied through a bypass circuit
  • Use of semiconductor control circuit retaining residual voltage
2. Abnormal relay conditions
  • Contact welding
  • Insulation deterioration
  • Mechanical failure
  • Induced voltage (long wiring distance)
Relay operation error.

Indicator light not functioning properly.
1. The voltage impressed on the relay input terminal may have exceeded the rated voltage.
  • Induced voltage (long wiring distance)
  • Bypass circuit from inductive voltage (A latching relay not holding.)
2. Excessive vibrations or shock may have been applied to the relay.
  • Poor operating conditions
Burnout 1. Possible coil burnout
  • Relay coil not suitable for the application
  • Voltage exceeded the rated voltage range
  • Imperfect operation of electromagnet with AC specifications (insufficient armature connection)
2. Possible contact burnout
  • Current exceeding the contact rating
  • Inrush current exceeding the allowable capacity
  • Short-circuit current
  • Poor connection with external components (abnormal heat generation due to connection failure such as with sockets)
Problems Visible from Inside the Relay
Failure events Checklist Possible causes
Contact welding 1. There may have been a large current flow.
  • Rush current e.g. from a lamp load
  • Load short-circuited current
2. There may be abnormal vibrations occurring in the contact component.
  • Subject to external forces (e.g., shock/vibration)
  • AC relay humming
  • Chattering noise made in the contacts when voltage drops causing faulty operation (the voltage may drop immediately after the motor starts running).
3. The relay may have exceeded its contact switching capacity (switching frequency too high). -
4. The relay may have reached its end of service life. -
Contact failure 1. Foreign materials may be on the contact surfaces.
  • Silicone, carbon or other foreign matter
2. There may be possible corrosion of the contact surfaces.
  • Contact sulphurization from SO2 and H2S
3. Contact failure may be caused by mechanical damage.
  • Terminal displacement, contact displacement, or contact follow
4. The contacts may be worn out.
  • End of relay's service life
Buzzing sound 1. Voltage applied may not be applied.
  • Relay coil not suitable for the application
  • Operating voltage fluctuation with the ripple factor
  • Input voltage rise slowly
2. The type of relay may not be properly selected for the application.
  • DC specifications used for AC lines
3. The electromagnet may not be working properly.
  • Foreign matter stuck between the moving armature and iron core
Abnormal wear of relay contacts 1. The type of relay may not be properly selected for the application.
  • Voltage, current, inrush current ratings do not match the application
2. Measures against surge (i.e. surge-absorbing element) need to be taken into consideration when switching the load.
  • Inrush current of motor, solenoid, lamp loads