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Water Ingress in Structured Cabling Systems
Provided by Molex Premise Networks
According to a recent
report from the Energy Savings Trust, London's future as an
international center for trade and commerce is under threat from the
risk of a major storm surge in the capital, which, it predicts, could
cause direct damage running to £20 billion ($31.6 billion). In the wider
Thames region, more than 150km2 (60 square miles) of land lies below
high tide levels. In the country as a whole, £222billion ($351.4
billion) worth of assets, including one in 13 houses in the U.K., is now
at risk of flooding.
Despite these figures and wide publicity surrounding the issue of global
warming and its highly visible effects on the U.K. climate in recent
years, many businesses have still not considered the effects of flooding
on their IT systems. There is seldom a winter month passes when the
company’s (Molex Premise Networks) technical support department fails to
get a call about the effect of water on the most vulnerable part of the
system: the structured cabling infrastructure.
This article looks at the effects of water ingress on data cables and
how different cables may be affected by flooding. It discusses the
issues that the IT manager and enabling installer should be aware of
regarding the possibility of flooding of the cabling system and what
measures can be taken to prevent damage.
Effect on Electrical Performance
The ingress of water into the data cabling system can have a serious
affect on its ability to support high bit rate data applications like
Fast Ethernet and Gigabit Ethernet. In some cases, cables that are
already near the link length limit (and therefore the attenuation
budget) for the standard may fail after flooding. This is because when
the cable gets wet, its dielectric performance is changed, which affects
impedance and the related parameters of attenuation and return loss.
Cable Construction and Water Ingress
It is a common misunderstanding that PVC, the material used to coat
standard data cable, is waterproof. It is not; it is hygroscopic.
Standard Category 5e and Category 6 cables have a PVC coating and are
designed for indoor use and are not suitable for use in wet conditions.
Cables designed for use outdoors, where moisture or water is present in
any quantity, incorporate waterproofing measures such as barrier tapes
and gel filling, which are costly and sometimes degrade electrical
performance. It is also worth noting that many of these waterproofing
gels are petroleum based and are therefore unsuitable for use indoors
(other than to a demarcation point), as they represent a fire risk.
Indeed, the distance such cables can be run within a building is usually
limited by national standards.
Further, the construction of the cable affects water ingress. Low-smoke
zero-halogen (LSOH) cables tend to have a lower resistance to water
ingress because the sheathing materials used are even more hygroscopic
than PVC. FTP cables, with a longitudinal foil screen, have better
resistance to water ingress as the screening material acts like a water
barrier tape. However, it should be noted that FTP is still not
waterproof.
Severity of Exposure
The
effect of water ingress is also dependent on where the water has been
and for how long. Short-term exposure to the middle of a PVC cable run,
with a small quantity of clean water, is unlikely to have any long-term
deleterious effect. On the other hand, if LSOH cables lying directly on
a concrete floor slab, with no containment or protection, are submerged
for a week, then the risk of damage is much greater. Water containing
dissolved contaminants from, for instance, a dusty screed floor, also
represents a greater risk.
An even greater threat than the effects of water ingress through the
cable sheath is posed by the open end of a cable being exposed to water.
The tightly twisted pairs inside a data cable encourage a capillary
action that can draw water significant distances into the cable,
destroying the electrical characteristics. Cable affected in this way
will nearly always have to be replaced.
Prevention is Better Than Cure
Replacing a wet cabling system is time consuming, expensive, and
disruptive. Preventing cables from getting wet is the best policy.
Ideally, businesses located in high-risk areas should consider the
threat of flooding at the network design stage. Measures can also be
taken for existing networks.
If it is possible, suspend cables in containment in the ceiling void.
This will be less susceptible to flooding than under-floor wiring.
Consider containment: Cable baskets offer better draining in the event
of cables getting wet, but closed containment provides better protection
from small amounts of water.
If cables have to be routed through the floor void, then position the
cable tray or basket as high up and away from the slab as is possible,
within the height constraints of the raised floor.
Because small floods caused by something as mundane as a burst water
pipe can be every bit as damaging as a large-scale river flooding, it
makes sense to keep wet services away from large concentrations of
cable, for example around telecommunications rooms and/or equipment
rooms. Don't run water pipes through these communications spaces, and
don't locate kitchens or toilets nearby. Avoid placing
telecommunications rooms or equipment rooms in basements—particularly
when adjacent to rivers.
If segregation is not possible, then consider feeding racks or cabinets
from above, and/or building bund walls to contain flood water within
less sensitive areas.
In the event that cables do get wet, the action required depends on the
number of cables affected, the balance of replacement cost, and the risk
to the business of not replacing the cables.
Cables in which water has entered via capillary action through the open
end should always be replaced. Cables that have suffered severe water
ingress through the sheath may also need to be replaced. Have the
network thoroughly tested and take advice from your manufacturer or a
trusted installer. If there are any doubts, replace the cables.
For more information:
Molex Corporate Headquarters
2222 Wellington Court
Lisle, IL 60532-1682, USA
Tel: 630 969.4550
www.molexpn.com
EMEA Headquarters
1580 Parkway, Solent Business Park
Whiteley, Fareham, England, PO15 7AG
Tel: 44 1489 572111
www.molexpn.co.uk
Asia
Pacific Headquarters
60-78 Abey Rd.
Melton, VIC 3337, Australia
Tel: 44 1489 572111
www.molexpn.com.au |
