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| Mains |
Incoming supply
The mains supply should be clean – not connected
to any other distribution that might affect it.
Large facilities may have the supply taken from
a dedicated transformer. We may even provide a
standby transformer for facilities where the risk
of damage to the mains incomer – accidental
or deliberate cannot be discounted.
Transient protection at this point will protect
the facility from the effects of lightning strikes
to the premises or the ground surrounding it.
Capacitors may prove to be cost effective –
the load on the facility will determine the
payback period.
The feed is connected – in the case illustrated
above, into a changeover cubicle, where an alternative
supply from a standby power source is also available.
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| Standby
Power |
For any reasonably
sized computer facility, the provision of an
alternative source of electrical power, in the
event of a mains failure, is essential. The
cost of such provision is small when related
to the cost of facility downtime.
For smaller facilities, the same comments apply,
although the relative cost may be higher.
Diesel generation is the most popular solution,
and gas turbines, running on similar fuel are
used to produce larger loads.
The generating equipment simply comprises an
engine, which drives an alternator, which produces
electricity.
A generator emits noise and exhaust fumes and
requires a supply of aspirating air for the
engine and fresh cooling air for its radiator.
It can be contained in a weatherproof, acoustically
attenuated housing and placed outside the facility
it is protecting. Alternatively it can be housed
inside a premises and the acoustic and anti-vibration treatments applied.
Fuel is usually held in a day tank built into
the frame supporting the generator. As the term
suggests, this will enable the set to operate
for
8 hours under full load conditions. It is usual
to provide a bulk storage tank to extend this
period. Fuel tanks need to be bunded to contain
leakage.
Permissions will be required from all relevant
local authorities and owners of the premises.
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| Mains
Changeover Cubicle |
Where there is
an alternative standby electrical power source,
we feed cables from it and the main provider,
into a changeover panel.
Inside are a changeover switch, sensors and
breakers. The changeover switch automatically
disconnects when a supply is lost and then switches
over when a supply is present.
The switch is mechanically interlocked so that
both supplies cannot be connected simultaneously.
Once thrown, it is usually arranged that it
be manually changed back to the original source.
Opportunity may be taken at this panel to split
the supply, where part is intended to be UPS
supported.
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| UPS |
UPS are the initial
letters of Uninterruptible Power Supply.
As the description suggests, its purpose is
to provide a source of uninterrupted AC electrical
power.
This is achieved by providing the power from
a battery bank.
A UPS contains an inverter to convert its input
to DC, a battery bank and a rectifier to convert
back to AC.
When the main source of power is lost, the battery
bank continues to provide power seamlessly.
When the batteries are no longer being replenished
by the outside electrical power source, the
UPS supply continues until the battery bank
has run down.
This expensive and limited resource is intended
to protect any equipment that would be damaged
or have adverse consequences on the business
activity, if its operation were interrupted
in any way. An example would be computer disc
drives. UPS support prevents them crashing and
provides time for operators to make backups
and close down in an orderly manner.
A UPS also gives time for any standby power
source to be brought on-line and take over –
in the event of a mains power failure.
This costly but effective facility needs careful
sizing and consideration as to what to connect
to it – which is the computer equipment.
We do not connect air conditioning, because
its loss for a few minutes will not affect the
room equipment. We do not connect the room lighting
– because sufficient fittings throughout
the room will be fitted with 3 hour backup batteries,
to enable operations to continue.
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| UPS
Bypass |
Whilst a UPS will
have a bypass switch inside it which enables
the unit to be switched off whilst being serviced,
we need a speedy means of isolating the UPS
when it is removed/replaced.
This facility is provided by means of an external
bypass switch. It provides an alternative route
for the mains electrical power, avoiding the
UPS.
The mains incoming electrical feed is fed through
this switch before connecting to the UPS. |
| Computer
PDU |
This distribution
panel is fed from the UPS – assuming one
is provided, otherwise it has a clean supply
and separate earth.
It may contain voltmeters and ammeters and sometimes
an emergency stop shunt switch.
From it emanate all the electrical power circuits
feeding the computer equipment and any others
requiring UPS support.
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| Emergency
Stop |
Requirement
Although all power supply cabling in a computer
room floor void, and the breakers protecting
them in the computer PDU, will all be clearly
labelled, this does not prevent accidents happening.
Access to underfloor connections is difficult
by definition of their location and the restrictions
caused by the packing density of equipment on
the floor.
Engineers are required to work in pairs, and
in the event of one receiving an electrical
shock – remembering that 400v 3-phase
is commonly installed – a means of immediate
electrical isolation may save life.
The large number of cables beneath a computer
room floor means that immediate identification
of the source of the incident is not possible.
Where this risk is considered to be present,
we fit a shunt trip to the computer PDU. This
immediately shuts off all supplies from it.
Location
The emergency stop button may be mounted on
the PDU and/or adjacent to the main entrance
door.
This is a drastic installation measure and needs
careful consideration.
Alternative
An alternative is investment in cable management
installation and maintenance together with strict
instructions regarding location and verification
of power supplies before works are carried out.
None however can entirely remove the risk of
an electrical accident.
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Mainframe
Computer Power Supplies |
Power supplies are
laid beneath the raised modular floor in steel
wired armoured cabling.
The armouring is necessary against physical damage
– especially prevalent with the presence
of the heavy steel encapsulated floor panels of
the floor above them.
Ideally each cable is labelled along its entire
length and secured to cable management trays.
This seems a sensible and straightforward requirement.
Unfortunately, in the majority of cases, this
does not happen.
The cabling may begin in that manner. However,
as soon as computer equipment is relocated, added
or replaced, problems are encountered which prevent
continuation of the ideal.
The causes are:
| a) |
Practical
Unless there happens to be an extremely
deep underfloor void, it is often found
to be impossible to access new and old cables
along their whole length because of the
other items of computer equipment preventing
tiles being lifted. |
| b) |
Human
With the best intentions, there are always
time constraints imposed. Downtime whilst
equipment is replaced and commissioned before
being brought on-line is at a premium. Engineers
working outside normal working hours are
not known for looking for the more difficult
solutions. Once one cable has been draped
unsecured across the floor void, others
quickly follow. |
This means that difficulties are found in both
releasing old cables and neatly installing new
into the cable management trunking. |
| Computer
Server Rack Supplies |
Small power supplies
emanating from the Computer PDU may be dealt with
via a network of trunking – from which supplies
are taken via socket outlets. This is has the
advantage of tidiness and clarity.
The area in which the racks are likely to be located
needs predicting beforehand rather than the installation
of general networks – the majority of which
are never used.
Where the supplies are required in an existing
computer room, the laying of such trunking is
usually impractical because of access disruption
and pollution. We therefore opt for the laying
of armoured cabling terminated in metal clad socket
outlets.
Because of the large number of these supplies
usually required, we bring the outlets from the
floor void and mount them on the floor tile inside
or near to the rack being served. From here the
multi-gang outlets can be safely connected.
A problem with the multiple single-phase supplies,
required in server racks, is phase separation.
We always need to balance the phase loading on
the Computer PDU, and in sufficient number, server
racks need to be fed from different phases. Because
racks are installed in rows and abutting one another,
there is the risk of phase mixing – 400v
– that can be lethal.
A 2m-distance phase separation may not always
be possible and engineers seem to be constantly
plugging in new equipment. It is therefore of
prime importance that supplies, cabinets and rooms
carry appropriate warnings and that the operations
department carefully monitor new installations. |
| Sundry
UPS Supported Supplies |
Other than the computer
equipment, there are other items of equipment
which should have UPS support:
i) |
Security items such as magnetic
door locks. |
ii) |
CCTV surveillance. |
iii) |
Fire detection control panels. |
iv) |
Services monitoring control computers. |
| v) |
Communications and network control computer
equipment. |
vi) |
PCs for external software development
and the like. |
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vii) |
The Operations departments PCs. |
viii) |
The IT manager’s and MD’s
PCs! |
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| Non-UPS
Supported Supplies |
| Equipment not requiring UPS support: |
i) |
Air conditioning
It is not necessary because in the case
of a mains power failure, by the time the
standby source has taken over, the room
conditions will not have detrimentally altered
sufficiently to cause problems to the computer
equipment.
Where there is no standby power facility,
by the time the UPS batteries have run down,
the computer equipment will have been powered
down – reducing the heat load as time
passes, and so no damaging room conditions
will have been achieved. |
ii) |
Lighting
A percentage of the light fittings in the
room will contain their own batteries, which
will illuminate them for up to 3 hours.
This is required by the Fire Regulations.
A failure of the remainder for a short period
is unlikely to affect the room operations. |
iii) |
Standby room heaters. |
iv) |
Non-essential equipment. |
v) |
Cleaners’ sockets – we do
not want vacuum cleaners plugged into the
UPS! |
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| Lighting |
The computer equipment
manufacturer may specify lighting levels. Some
are satisfied with the usual 600 lux, others
demand 1000 lux on the basis that it assists
their service engineers.
Recessed light fittings are fitted as they reduce
airflow restriction.
Low glare is achieved by fitting parabolic grids
to the luminaries.
A level of background illumination should be
maintained in the event of a power failure –
Fire Regulations.
Apart from illuminated escape signs above the
exit doors, we can fit either mains maintained
battery packs into some of the luminaries, or
fit separate light fittings containing batteries.
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| Earthing |
It is necessary
to ensure that the mains feed into the room
has its own clean earth.
The whole of the room installation needs to
carry earthing in accordance with the 6th Edition
of the IEE Regulations.
Pedestals supporting the raised modular flooring
need earth bonding.
The computer installation engineers are responsible
for earthing their equipment.
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