# How does the high inrush relay handle the inrush current?

I was searching through the high power relays(16A, 240AC) and found that some manufacturers have high inrush capability relays and that too upto 100A inrush current. Below are some examples (one is latching and other one is standard)

Can someone help me to understand how these relays would handle the inrush current ? Some internal schematic if available ?

Also since these relays have 100A inrush, can they not easily work with 1HP (240VAC) pumps?

• What do you mean by "handle" exactly? It is just the current that is allowed to go through it for a brief moment (when cool) May 18, 2018 at 14:15
• Normal relays would weld their contacts if high inrush current is passed while turning ON, where as these are expected to overcome such cases. I am trying to understand how!
– Zac
May 18, 2018 at 14:18

Inrush capacity just means that inside the relay, the components are capable of dissipating the heat caused by the very brief (1-2 cycle) inrush current without causing damage.

That has nothing to do with the "breaking capacity" of the relay. Rating of a switching device for control of a motor is not really about the inrush, it is about stopping the flow of current when the motor has been running. When contacts separate under load there is an arc that forms, and it is extinguished when the dielectric of the air is sufficient to stop the current flow. Induction from the motor will cause the voltage to rise in the arc as it forms, which will sustain the arc for longer. The arc is then melting the contact surface areas, potentially destroying them. So the capacity of those contacts to stop that current flow and extinguish that arc without vaporizing and/or welding in the process, determines the maximum motor size it can handle. If you are switching a motor, you need a relay / contactor that has a motor rating equal to or greater than your motor power (HP or kW). If the contacts have no motor rating, they are incapable of being used in that manner.

• That makes sense, I never thought about the stopping flow of a motor. For a 1HP 240VAC motor, what should be the relay rating? Is 16A sufficient or are there any other parameter which I should be evaluating ?
– Zac
May 19, 2018 at 6:13

Schematically, all relays are the same. The current carrying and switching capacities are determined by the dimensions of the internal parts and the materials used. The capacity to conduct and dissipate heat must be determined for each piece. The capacity for extinguishing the arc when breaking current must be analyzed. The effect of contact bounce must be analyzed. The heat produced the arc and by current flowing in the parts must be analyzed. The spring force vs. the solenoid force, the geometry to the parts and other details of opening and closing the contacts must be analyzed.

If you open up two relays of similar physical size but different current and voltage ratings, you can see the different dimensions of the internal parts and perhaps differences in design details. You are not likely to find detailed dimension drawings and design calculations for specific products. You may find some design examples in a text book.

• I had few normal relays rated 10A (240VAC) which got welded when connected to the power supply of an LED strip. I hope the inrush models would survive such cases as advertised.
– Zac
May 19, 2018 at 6:16

High inrush current is handled by a combination of contact pressure and other mechanical design attributes plus metallurgy of the contacts. In the case of relays with a high-inrush version where all other specifications (coil power in particular) are similar, chances are that the only difference is the contact materials.

The main issue with inrush current is that an excessive current can cause the contacts to weld together, so that they do not open when the coil is de-energized, which is very undesirable. Sometimes when this happens you can tap the relay and they will open, only to weld again the next time the load is switched on. Wear is an important secondary concern. The contact materials that resist welding are not typically the best for switching other types of loads such as resistive or inductive.

In the olden days cadmium oxide alloys (AgCdO) were used, but these days less toxic materials such as Silver Tin Oxide (AgSnO2) and Silver Tin Indium (AgSnOInO) and Silver Copper Nickel (AgCuNi) are used. Here is a document from TE which describes a plethora of contact materials.

Contacts that are optimized for high inrush current handling will likely have some other facet of performance or cost that is sub-optimal (such as breaking ability for inductive loads) so there is no one contact alloy that is good for every application. Note also that if you whack the contacts with huge inrush currents, they will get hot from I^2R heating and if you repeat the performance without enough cool-down time, the relay can be damaged.

The inrush current can be specified for I2t pulses and not just for permanent RMS currents. DHR circuit breakers are also, as relay, specified in the main train lines for overloads. See the triac specifications for information on the curves giving the peak current pulse for a given pulse duration. The relay does not handle the inrush current, but it can support it ... if well sized. You need to know how inrush current can be.