Are you working on your IT infrastructure? You need a clamp meter.
It’s a truism but all that we do in IT starts with the electricity supply. It feeds the servers, the network, the air conditioning and even the vending machine in the lobby. So when designing or re-designing any server room or data centre you really need to know how much of that invisible go juice that you are using.
Recently I have been checking the power usage of our Disaster Recovery (DR) site in order to ensure that it is appropriately provisioned with UPS (Uninterupterable Power Supply) devices. The problem with checking a live site, even if it is a DR site, is that the servers and services are on-line 24/7 and cannot be switched off. So you need to get something that measures the power being consumed without interupting the power flow and thus requiring the servers to be shut down.
So enter Electro Magnetic Field (Wiki Link), or EMF for short, which is a field generated by electricity running through a cable. Now quite how that field works is something i ended up doing at university when studying Electrical Engineering and believe me it isn’t that interesting. What it is though is a both an annoyance and absolutely vital for our current method of delivering our electrical infrastructure no matter where you are.
The electricity running through a cable generates a field in direct proportion to the current running through it. It also generates that field with a rotation much the same as water flowing down the bath plug. The rotation is affected by the direction of current and if you reverse the current then the direction of the field changes. If you run the exact same current in opposite directions at the same time (i have no idea how you would actually do this) then the two fields would cancel each other out.
So, back to measuring power use.
With Direct Current (DC) the current flows only in on direction at any one time. Except we don’t use DC we use Alternating Current (AC) which changes direction continuously; actually it changes direction 50 times a second (ever wondered what Hz meant?). Our use of AC current means that the field will be fluctuating back and forth as the field changes direction every 1/50th of a second making measuring more challenging than you would think :)
Manufacturers have countered this problem by having an instrument that switches round its measuring at the same rate/frequency as the AC does (lets say 50 times a second to be clear). You can pick these up from places like screwfix (http://www.screwfix.com/p/kewtech-kt200-digital-clamp-meter-400a/23318#) but in order to work you need to measure a single wire….
Eh? A single wire? Yes because in any electricity cable there are 3 wires; 1 live, 1 neutral and 1 earth. Forget about the earth for the time being but you can think the live and the neutral as “it flows up one and down the other” and if they are next to each they are always flowing in the opposite direction to each other thus cancelling out that field you are interested in measuring.
So what if there is a big cable and you are not going to be able to cut into it to measure the single wire? Ah that’s when you start looking at more expensive products. These are engineered to realise there is a seperation between the wires and measure the current based on that difference. In other words; they are very clever products. I picked up a clamp meter from Megger to do the job i needed to do:
(Megger MMC850 Multi Core - Single Core Clampmeter)
You can see the dials on the clamp meter to let it know what type of wire configuration it is measuring and then how much current is being drawn through that cable (7.4Amps in this case). The wire is held in the clamp at the top and never needs to be un-sheathed from it’s protective housing or generally interfered with.
Of course you can use it for smaller cables such as this general mains (13Amp) cable where it is reading a heady 2.8 Amps!
Really a clamp meter is about understanding what your basic power requirements are in a live environment. Your power usage also will help you work out what your air conditioning requirements are where the amount of power put into a set of equipment is directly related to the amount of cooling you will need to dispose of the heat.
Get this sort of stuff wrong and you could end up blowing fuses or melting your servers!
