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I'm working on a switching device and I'm thinking of switching a hotel room lights + aircon + heater + tv, this is my my project for school. The Main Switch says 100A.. can a solid state relay of 40A sufficient to control that? I'm wondering why others sells 25A magnetic key car switch. Please help

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    \$\begingroup\$ A 40A switch is not enough to switch 100A, but might be enough to switch whatever's in the room. You need to start from the load ratings and then decide what size switch you need. You would not normally control an A/C unit by interrupting the power to it; if you're going to do that then you need to be careful that you don't upset the A/C equipment in any way. \$\endgroup\$ – user1844 Nov 24 '16 at 8:37
  • \$\begingroup\$ And if you will have anywhere near 40 A on the SSR -- keep in mind that it needs a serious heat sink. They drop about 2 V. So that means it could be up to 80 W. That's a lot of heat generation. \$\endgroup\$ – jonk Nov 24 '16 at 8:48
  • \$\begingroup\$ I'm wondering how come they are selling 30A keycard switch. any reason to that? \$\endgroup\$ – E B Nov 24 '16 at 8:51
  • \$\begingroup\$ Not an SSR? Probably using a relay. A 120VAC relay uses about 2 W of power. \$\endgroup\$ – jonk Nov 24 '16 at 9:04
  • \$\begingroup\$ The unit I designed here for true 40A switching uses a combination of a relay AND an SSR. The SSR comes on for 0.1 second, while the relay engages, to protect the relay contacts from arcing. Then, once the relay is fully engaged (at 2 W power), the SSR is turned OFF. The SSR is only used for short moments in order to improve the relay lifetime by protecting the contacts from arcing. So I don't need much heat sinking for the SSR (which uses pairs of SCRs instead -- even better.) \$\endgroup\$ – jonk Nov 24 '16 at 9:07
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The size of main fuses is always for protection of the supply chain and is chosen far over the maximum requirements of your local system. In case something goes very wrong with your local circuits the main fuse blows to prevent a supply problem with adjacent users not connected to your circuit.

Example: The mains fuse of your house prevents that in case something goes wrong in your house does not cause a problem in the house of your neighbours.

Now back to the hotelroom situation. Keycards are used in hotels, etc. and are layed out for mostly a little more then the requirements.

So if the hotel keycard has a switching capacity of 30A then an SSR of 40A could handle it as well. But a warning here is relevant. The SSR loses about 2V. Therefore the power dissipation could become 60 or 80W so the use of a large heatsink is needed.

A standard keycard system remains therefore much less expensive then a system with an SST in the power chain.

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  • \$\begingroup\$ @EB If you think that my answer is usefull then please upvote and accept. \$\endgroup\$ – Decapod Nov 24 '16 at 9:46

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