The Computer Lab has high accuracy electricty monitors installed on main circuits within the building. These meters are Autometers' IC995, and provide overall power usage data, as well as the ability to measure additional data such as powerfactor and harmonics. These provide power information for the main circuits of the lab, such as electricity use for lighting per corridor.
We also have data from our electricity provider utility on electricity use for the entire building. This is reported to us, and made avalible at the top level of the meter tree.
Low Cost Meters
The USB current monitors developed at the University of Cambridge Computer Laboratory provide a low cost means of monitoring a large group of circuits, for example groups of servers in a machine room. In the first version cach monitor connects to one current transformer and one USB link. The monitors are powered over the USB connection and have no other power supply.
For a collection of circuits, monitors are connected via USB hubs to a linux box. Power is required for the USB hubs and the linux box. The hubs are connected (either to the linux box or other hubs) with 'standard' USB A to B cables; the current monitor boards are connected with A to micro-B cables.
Each monitor has a serial number, which is programmed into the device. This is how the monitoring software makes the correspondence between circuits and readings the linux box requests.
Multi Circuit Meters
As we have been asked to provide metering infrastructure beyond the project, we have developed a monitor that is more suited to this multi-circuit configuration. This has resulted in a single USB device which can monitor up to 32 circuits (thus eliminating the need for any USB hubs). This is known as Mon32 and is pictured below (at the bottom of the page) and is described more fully here.
The photographs below show an installation in a server room at the Computer Laboratory at Cambridge using the earlier version of the USB meter.
Figure 1 shows four current monitor boards housed in a single case which is mounted on a board. The current transformer tails (black and white wires) are connected on the right, the micro USB connectors are on the left.
Figure 2 shows the configuration in construction. 24 of the transformers are connected into the monitors, approximately 16 others are not yet connected. Under the main distribution panel there are two Autometers which provide overall power (and voltage and power factor) readings.
Figure 3 shows the panel opened out so that the USB hubs on the back are visible. (The linux box is not yet mounted on the panel and can be seen on the floor. It is about the size of a USB hub.)
Figure 4 shows the back of the panel, in particular, one of the USB hubs. The power connector and link to the linux box are on the left; the other USB links are to the monitors on the other side of the panel.
Figure 5 shows Mon32. The board is approx 100mm sqaure and consists mainly of connectors for current transformers.