Annex O (informative) — Heat and mechanical load
The heat a row produces, how a case gets rid of it, and the forces a panel and its screws carry. EIS places no requirement here; the figures are planning aids built on the currents modules already publish (PWR-001).
O.1 Heat from a row
Every watt a module draws from the bus becomes heat inside the case. From the published currents, a module dissipates at most
P = 12 V × I(+12 V) + 12 V × I(−12 V) + 5 V × I(+5 V)
and a row dissipates the sum over its modules. A published current is a maximum, so the sum is an upper bound. A supply inside the case adds its own loss, P × (1 / η − 1), where η is its efficiency at that load.
A worked figure, for one 84 HP row whose modules publish 1.2 A on +12 V, 0.40 A on −12 V, and 0.10 A on +5 V:
| Source | Calculation | Watts |
|---|---|---|
| +12 V | 12 V × 1.2 A | 14.4 |
| −12 V | 12 V × 0.40 A | 4.8 |
| +5 V | 5 V × 0.10 A | 0.5 |
| Modules | sum | 19.7 |
| Supply loss at η = 0.85 | 19.7 × (1 / 0.85 − 1) | 3.5 |
| Inside the case | 23.2 |
A linear power converter that makes PWR-A6 from the bus adds its own loss: (12 V − 6 V) × I on each rail. One feeding a module at 0.3 A per rail turns 3.6 W into heat behind the module, as much as the module's own ±6 V load; it publishes the figure (SUP-008), and the row budget counts its input current on the bus.
O.2 How warm the case gets
A closed case loses heat mainly by natural convection from its outer surfaces. A rough estimate of the rise of the air inside above the room is
ΔT ≈ P / (h × A)
with h about 5 W/m²·K for still air on painted metal or wood, and A the outer surface area. A 3U, 84 HP case about 0.48 m wide, 0.15 m deep, and 0.15 m tall has about 0.33 m² of surface, so 23 W raises it by about 14 °C. The same row in an open-backed or vented case runs much cooler.
EIS recommends that an enclosure publish, for each row, the dissipation it carries for a 10 °C rise above room temperature, and that a module whose dissipation is much higher than its current suggests, such as one with a linear regulator dropping 12 V to 3.3 V, say so in its manual.
O.3 Mechanical load
| Load | Planning figure | Where it comes from |
|---|---|---|
| Plug insertion at a jack | 10 N | The force Annex J uses for panel bow |
| Mounting-screw torque | at most 0.5 N·m | An M3 screw holds in three threads of a 1.5 mm to 2.0 mm insert (5.3); more strips the insert before it tightens the panel |
| Module mass | publish it when it is more than the panel and board alone | A heavy module on a portable case loads its four screws in transport |
EIS recommends that a module whose mass would bend its panel in transport, such as one with a large transformer or a display, publish its mass and supply a support bracket where it needs one.