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I switch off the power bar which has my modem and computer plugged into it a few times per day. Two days ago, a technician changed the wiring since my Internet completely stopped working for the second time.

I am wondering if turning off the power bar is damaging the cables or modem and is what caused the problem.

Can shutting off a modem directly from the power bar damage the cables or modem?

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Shutting off the power to such a device should not hurt it. After all, power can be interrupted for various reasons like a power outage, a breaker popping, and the like, so devices are designed for that.

Some complicated devices may have software issues when power is suddenly interrupted, but should not suffer hardware failure. For example, you shouldn't suddenly shut off power to a PC since that doesn't allow the OS to flush pending file changes to disk, allow device drivers to shut down their devices in a orderly fashion, etc. The PC will physically survive, but some files may be corrupted.

Routers and modems without mass storage devices are much less likely to have software issues with suddenly interrupted power, and should not get damaged by that physically. Cables are impossible to damage just by interrupting normal power. To kill a cable you have to put significantly more than the rated current thru it, which interrupting power doesn't do.

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  • \$\begingroup\$ Modems are often managed devices. This means that the internet service provider can be doing all sorts of tweaking, configuring or monitoring whenever they feel the need for it. Eg. they can configure settings that are required to operate with a new central system. Powering off these devices may increase the probability for an critical update to be missed. \$\endgroup\$ – jippie Oct 25 '12 at 17:55
  • \$\begingroup\$ Systems like you describe @jippie generally have version requests etc. that notify a backend that it needs an update. It would be a pretty poorly designed upgrade server if missing one of the upgrades prevented the device from any further communications. \$\endgroup\$ – NickHalden Oct 25 '12 at 18:01
  • \$\begingroup\$ @NickHalden but what happens if the device is power cycled while the device is flashing new firmware or storing configuration in EEPROM? Many devices have a critical period during upgrade or configuring that it's better to not loose power. \$\endgroup\$ – jippie Oct 25 '12 at 18:07
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    \$\begingroup\$ @jippie: For any well designed device, this critical period is either very short or non-existant. For a short one, the power supply bulk storage cap can run the system long enough. You don't just upload new code over the old code. You save the new code in a separate memory or memory region. The bootload that runs at powerup does the appropriate copying and checksum tests, and never runs the main app if its checksum fails. Presumably the bootloader can't be overwritten. \$\endgroup\$ – Olin Lathrop Oct 25 '12 at 18:46
  • \$\begingroup\$ @jippie Yeah mostly what Olin said. If the checksums don't match what it expects after an upgrade it will use the part of the code base (bootloader and comm section) to request some sort of fix... whether that means attempting to intelligently fix a portion of the code or just re-doing the whole upgrade is dependent on the device, but it would be real bad if losing power for even a femtosecond left a device bricked. A well-designed device has many protections from this. \$\endgroup\$ – NickHalden Oct 25 '12 at 20:05

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