Playing Russian Roulette
Many racks do not have metered PDUs. This is like playing Russian roulette, since you have no way of knowing if the next server you add to the rack will kill it! The only way to safely implement a dual-server PS and dual-rack PDU is to never exceed 40 percent of the face rated value of the rack PDU or path. All circuits must always be protected by a circuit breaker. The UL- and NEMA-mandated codes require that you can only safely draw 80 percent of the rated value.For example, you cannot draw more than 16A from a 20A PDU. This means that in a dual-PDU rack, the entire equipment load should not exceed 16A for the rack. Therefore, each PDU should normally only have an 8A load on it, to avoid a potential cascade overload and resultant rack-level power failure. As mentioned earlier, even those administrators who do have metered PDUs, do not realize that once they go past the 40 percent power level that they are in danger of have a cascade power failure. Moreover, as servers are upgraded and added all the time, it is easy to see how the exposure continues to increase with no warning, until a problem occurs and then everyone involved is baffled why power was lost because everyone thought they had "redundant" power.