highly reliable

Hybrid Relay Switching

The conventionally switched Power Distribution Units (PDU) 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 the electro-mechanical relay is its poor reliability due to arcing. The mechanical contact in the electro-mechanical relay could produce an arc when the contact breaks the load current, which could lead to reduced relay lifetime. This arc could increase resistance of the contacts over time, which may even lead to welded contacts!

On the other hand, the main problem with the solid-state relay is the heat it produces. A solid-state relay wastes power in the form of heat due to nominal voltage drop across the semiconductor switching path. For instance, a solid-state relay can consume more than 15 watts of power for a 10A load. This not only wastes power; but it also increases the cooling cost if you have many PDUs that use solid-state relays. But a hybrid relay, such as the one used in Echola Systems-switched PDUs makes use of both electro-mechanical relay and solid-state relay technologies to eliminate the disadvantages of each, while at the same time providing extremely reliable switching.

To better understand the concept of hybrid relay switching you can look at the steps involved in switching ON the outlet in the following illustration.

hybridrelay.gif 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 the contact closes to make a parallel path. After a period of some delay, usually until the contact bouncing of the EMR settles down, the control signal to the SSR is removed. This makes the EMR handle the entire load by effectively bypassing the 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 the 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 the SSR only when switching, which means that the switch itself consumes much less power. For instance, the SPDU-108M model consumes less than 3 Watts of power when all 8 outlets are in the ON state!

Thus, the 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 increase the cost of the switched PDU. But Echola Systemsí smart PDUs use patent-pending shared hybrid relay switching technology, which reduces the overall cost of multi-outlet switched PDUs yet provides the same benefits of a hybrid relay.

The following video shows how hybrid relay technology used in Echola Systemsí smart PDUs helps to switch load without producing any electrical arc when compared to other PDUs. Even the zero crossing technique used in many next generation PDUs won't completely suppress arc in EMRs due to its longer switching time (make or break and bounce time). We used an APC PDU, AP7901 with a load current of about 4A @ 120V AC to create an arc and compare the result with our SPDU-108M PDU.

This video also demonstrates how outlet level overload protection can help to reduce system downtime. Click on the video to start watching. You may want to select full screen option with 720p or 1080p quality for better clarity.