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Power over Ethernet for HVAC/metering: A Success story

September 15, 2014

We have a large campus customer site where we do metering for electric, steam, chilled water and water (most of WAGES except gas – and that is on the roadmap). 

The meters of interest all have Modbus protocol capability – either as an easy switch in their interface, or as an added inexpensive board from the meter manufacturer, or as a simple add on box from the manufacturer or a third party (a surprise overall for the customer).

But once Modbus enabled, they need to get into some TCP/IP protocol for the enterprise aggregation. Now TCP/IP can be carried over many data-links, but WiFi and Ethernet are certainly the most common and easiest/cheapest with which to connect.

Installation of boxes direct from Modbus to BACnet/IP and Modbus TCP facilitate the connection to TCP/IP… Boxes like Cimetrics B6030 Modbus Device to BACnet/IP edge device and Cimetrics B2000 Modbus MBAP RTU/TCP router. Direct pulse pickup is done by our Cimetrics B6070.

The Cimetrics Eplus devices are Ethernet boxes which need power to run and drive RS485 for Modbus RTU and I/O for pulse. For some commodities, especially water, the installations are hard to bring electrical outlets to, and even with in house labour involve some expense (around a few hundred dollars). They are also costs in bringing Ethernet, but this compares to some locations (like water and sewer vaults) to which WiFi is impractical to bring. The cost of the actual box placement combined with power and Ethernet installation becomes greater than the cost of the box itself.

During the campus install the IT department came to us (they were installing Ethernet drops) and said:

What are you doing? Don’t you realize that we have a global roll out of 802.3af for VOIP and security services and power for your meter device ports is sunk cost in existing infrastructure – there for the asking (and helpful to use as forward going justification of that Power over Ethernet(PoE) infrastructure install) . Even if we did not have it, in locations where you deploy your own switches/hubs – why not use (cheap) 802.3af enabled gear to get your power?

We felt a bit foolish given our IT expertise, but immediately changed tracks and stopped requesting 120VAC power outlets, instead submitting a request for 802.3af along with our request for a drop and IP address. Forward going install costs were halved. We were able to do this in part because our devices are low power and fit well within the 10W 802.3af PoE limit. Ongoing tracking and maintenance were improved because the IT plant was the power provisioner too.

It was such a good arrangement that we instituted a systematic programme to get 802.3af capability for all our small box Eplus products. Sometimes this capability is a simple flexible 802.3af injector. We can provide integrated 802.3af as well, for large installs which have 802.3af as the main method of power.



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