Power Supply Issues:

There are two main types of power supplies that are used with an industrial chassis; a rear mounted case supply, or a front mounted card style power supply. The most common power supply style is the enclosed case type which is mounted in the rear of the chassis behind the backplane. Most case style power supplies will be the switching type which are smaller and lighter than linear power supplies. All power supplies in PC cases are switching power supplies. Most are shipped with Fans attached to the power supply case.

The card style power supply may be used in many types of backplanes. Most board specifications which define a card and backplane combination allow for a card slot to accept a power supply board. The cPCI board standard is one of many standards that allows front inserted power supply boards. The trade-off in using the board style power supply is the loss of one or more card slots in the chassis.

The power supply should be sized to supply the required cards and [DC] fans within the chassis. The power supply may also need to be sized to power empty slots, so cards may also occupy those slots if required. Spare slots should not be provided if they can't be power, or blocked off so they may not be utilized.

For operation above 50 degrees C the supply should be de-rated 20% for each additional 10oC above 50oC or 2.5% / oC rise [as a guideline]. Of course always consult the devices data sheet to determine the amount of derating required.

Power Supply Specifications and Standards next page

Safety Issues;
1. Overloading or improper cooling of power supplies can cause excessive temperature rise.
2. Output circuits and components can remain energized after input power is interrupted.
3. Provide to personnel a visible indicator that the power supply is energized.
4. Provide protective covers and/or barriers over terminals and other energized parts to protect personnel.
5. Apply suitable marking to identify the hazard at the power source and at appropriate places.

How To Specify a Equipment Chassis index page

Current overload protection. Current overload protection should be provided for primary circuits. Devices such as fuses, circuit breakers, time delays, cut-outs, or solid-state current-interruption devices should be used to open a circuit whenever an overload condition occurs.

No over-current protective device should be connected in series with any conductor which is grounded at the power source unless the device simultaneously opens all load conductors in the circuit and no pole operates independently, or as otherwise allowed by the National Electrical Code, NFPA 70.

Protective devices for wired-in equipment should be connected to the load side of the equipment power switch (main circuit power disconnect).

For portable equipment a separable connector or the attachment plug and receptacle should serve as the main circuit power disconnect and the protective device may be on either the line side or the load side of the equipment on-off switch.

PC motherboard

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