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Supplying you Infrastructure with Power

By kevaburg ·
Introduction

Many organisations spend thousands on ensuring that their IT infrastructure is the best that it can be. Servers, workstations, redundant network circuits and advanced routing protocols all ensure that the business requirements of an organisation can all be met but there still remains a single-point-of-failure (SPoF) and that is the power that runs all of this.

This article aims to outline the basics for designing reliable power supplies from the building intake to the end user.

The Basics

Power is generated at the power station in three-phases. In the UK these are coloured red, yellow and blue. The neutral is black and the earth normally yellow or green. Why it is generated in three-phases is beyond the scope of this article but the reason it is important to know this will become apparent.

Each phase has the potential for two different voltages. The first is the voltage generated between a single phase and neutral. This connection is called ?star-point? and each individual phase connects to the neutral but not to each other. The output voltage in this instance is 239v.

The second is known as ?delta?. This configuration has each phase connected to the other two so that on paper, the shape formed is a triangle. Because the phases are connected together, 415v becomes generated.

Now consider this: A row of houses, 6 in number are all connected to a single-phase. The load on that phase would be considerably more than on the other two and this situation could cause an unbalance resulting in the power failing over all three phases.

The same six houses now utilise all three phases. The first house uses the red phase, the second yellow and the third blue. Houses four, five and six follow the same pattern. Now what happens is that the general spread of power is almost equal throughout the three phases. There will be the occasional power spike when heating comes on or a kettle is turned on, but generally speaking the power levels remain even.

How is this relevant to our IT?

The Curse of the Coffee Drinker!

Like most IT specialists, I drink a lot of coffee. I also like to listen to the radio while I work and my computer is plugged into the same circuit. If it is cold then I will have my portable heater plugged in. I come into work in the morning and my computer is turned on. While I am waiting for it to boot, I turn the kettle on. The heater has been turned on via the thermostat. At this point there is a power surge at the heating element kicks in in both devices and the amount of current required is greater than the circuit is designed for. The read-write heads in the harddisk fail and crash into the platter, the surge causes damage to my memory, processor and motherboard and instantly, the once useful tool with all my data on it has been converted to scrap.

This isn?t an unrealistic scenario and I would imagine many people would have witnessed it in one form or another in their own environments. After all, if you move into an office for the first time, do you really ask if there are only certain sockets to use for anything other than IT?

Power Distribution

In the event of a power failure there are large organisations or organisations that have mission-critical power requirements (such as hospitals for example) that have generator farms to continue power even when the mains is down.

This isn?t practical for everyone due to the sheer expense but there are some guidelines to follow that can make everyones life better.

1. Create a different 3-phase supply source at the building intake and designate it for IT use only. In each server room or wiring closet, install 3-phase distribution units for distribution to smaller circuits.

2. From each distribution board, designate a circuit specifically for workstations.Each socket that a printer, computer or scanner plugs into should be coloured red and users made aware of the significance of the colour. Nothing else should be plugged into it.

3. The remaining two phases could be distributed evenly between groups of servers, RAID-arrays or other hardware. If for example you have two AD-domain controllers, place one on each phase. A failure in one phase will still ensure the server on the second phase is protected.

4. Don?t ever neglect or underestimate the value of a good UPS! Even though we have provided some limited protection by spreading our IT across isolated multiple phases, this still does not protect us from the power spikes and surges the likes of which can be experienced through lightning strikes. A UPS can protect us from the negative effects of overload conditions, power surges and spikes. Unfortunately, the cost of these devices precludes the use of one per workstation, but in a server room or wiring closet they can prove invaluable.

6. Document accurately the distribution of each electrical circuit, where it runs and who and what is services by it. This will enable a fault to be easily isolated and prevent normally unaffected users from being affected unnecessarily. If we have spread our IT resources, workstations, servers, routers evenly over three phases it is possible to close one phase down and allow anyone else on the other two to carry on normally.


Calculating Power Requirements

How exactly do you calculate the power consumption of a single device so it forms the basis of useful information for the distribution of our power? Ohm?s Law.

We use the formula: P=VI where P=power in watts, V= voltage (normally a constant) and I=current.

All electrical apparatus has its electrical requirements somewhere on it although normally it is stamped or printed on a metal or foil plate somewhere. We have already established that one of our values is a constant; voltage and in the UK this is a round 240v. If the device consumes 0.5A maximum (and maximum values should always be assumed in these calculations) then by Ohm?s Law the device delivers 500W of power. This article does not cover apparent or true power for the ease of translating Ohm?s Law?s results.

So what do we know from this?

If we add all the total maximum currents for each of our devices, we can ensure that it doesn?t exceed the level of protection in our distribution boards. This helps to prevent overload conditions.

In totalling the power in Watts for each device, we can ensure that we don?t exceed the level of protective risk offered by our UPS.

What if we still have unequal phase loads?

Simple. If we find ourselves with two circuits equally divided and well within the limits of the protective equipment at the distribution board, then we can move equipment from one circuit to another. Try to maintain redundancy with regards to critical hardware such as servers but distribution across all three phases will help to ensure reliability of your power supply.

Summary

The configuration of your power supply requirements is as important as any other part of the network design. These suggestions are by no means the be-all and end-all of power redundancy but they will help to go some way to ensuring reliable power for a companies IT.

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Different in the US

by TheChas In reply to Supplying you Infrastruct ...

Apparently, in the US the power grid is structured a bit differently (and more complex) than in the UK.

Depending on the physical location of your facility, you might have the choice of several different forms of power:

240 volt single phase.

208 volt single phase as derived from 240 volt 3 phase.
(208 volt single phase is derived by connecting power across 2 legs of a 240 volt 3 phase line.)

208 volt 3 phase.

240 volt 3 phase.

440 volt single phase.

440 volt 3 phase.

(In some areas of the US, other voltages exist for 3 phase power.)

Nearly all residential power is 240 volt single phase.

In an office park that is zoned for only offices and light industrial, 240 volt single phase may be your only option.

Even in a building that has 440 volt 3 phase power, that power is usually on a dedicated circuit and used to power only factory equipment.

You still normally have a 240 volt single phase circuit for all office equipment and user outlets.

Load balancing is still worthwhile in the US as our 120 volt outlets are created by splitting the 240 volt single phase in half.

Either side of the 240 volt feed when referenced to neutral provides 120 volts.

When you connect across the 2 hot leads, you get 240 volts.

So, you do want to design your power distribution to equally load both sides of the 240 volt feed.

Aside from the potential reliability benefits of equal power distribution, the closer the load on both sides of the feed is to balanced the less power is lost in the step down transformers. Also, a balanced load results in a slightly lower power bill as power meters look at the peak current draw on the line.

In the building I work in, we also have locally generated 115 volt 400 hertz 3 phase.

Chas

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