The conventional switched PDUs in the market use either electro-mechanical relay (EMR) or solid-state relay (SSR) technology for switching the outlets. While solid-state relays are more reliable than electro-mechanical relays, the electro-mechanical relays are much cheaper and consume less power than solid-state relays. But the main problem with electro-mechanical relay is its poor reliability due to arcing. The mechanical contact of electro-mechanical relay could produce an arc when it controls a fairly high voltage and large current flows that could lead to reduced relay lifetime. This arc could increase resistance of the contacts over time which may even lead to welded contacts!
The main problem of solid-state relay on the other hand is the heat it produces. Solid-state relay wastes power in the form of heat due to nominal voltage drop across semiconductor switching path. For instance, a solid-state relay can consume more than 30 watts of power for a 20A load. This could not only wastes power but also increases the cooling cost if you have many PDUs which use solid-state relays. But a hybrid relay such as the one used in Echola Systems switched PDUs on the other hand makes use of both electromechanical relay and solid-state relay technologies to eliminate disadvantages of each, at the same time provide extremely reliable switching.
To better understand the concept of hybrid relay switching you can look the steps involved in switching ON outlet in the following illustration.
The hybrid relay output consists of a SSR in parallel with an EMR. The SSR output switches the load when the control signal is applied. Once load-current is established, the EMR energizes and contact closes to make a parallel path. After period of some delay usually until the contact bouncing of EMR settles down, the control signal to SSR is removed. This makes EMR to handle entire load by effectively bypassing SSR until the outlet is switched OFF. Since the SSR is bypassed by the EMR during normal operation, there is little or no heat build-up inside the relay. This allows the hybrid relay to operate without a heat sink. Moreover, since the EMR energizes without voltage across the output contacts (since SSR is handling load in parallel) there is no electrical arcing on the contacts.
The switching OFF of outlets is done in similar way to avoid arcing.
As explained above, Echola's smart PDU uses SSR only when switching which means the switch itself consume very less power, for instance the SPDU-108M model consumes less than 3 Watts of power when all 8 outlets are in ON state.
Thus combination of SSR and EMR provides an extremely reliable relay capable of switching heavy current loads. Implementing such hybrid relays for multiple outlets would certainly increae cost of switched PDU. But Echola Systems smart PDUs use patent pending shared hybrid relay switching technology which reduces overall cost of multi-outlet switched PDUs yet providing same benefits of an hybrid relay.
