Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

Alphanumeric Time Domain Reflectometer

The TDR has been around for some time.

Widely used by the military, computer net-

work technicians, and the telecommunications

industry, the technology is proven and accepted

as the most efficient way to trace problems in

long wire runs. Historically, these devices have

40 electrical systems troubleshooting

been expensive; at prices ranging from $5,000

to $10,000, they were beyond the means of

almost all marine electricians, and out of the

question for boatowners. A few years ago, how-

ever, costs dropped dramatically, and good

alphanumeric TDRs became available for

around $500. I immediately tested and subse-

quently purchased one of these new-generation

units, and I’ll never go back to using an ohm-

meter for tracing wire runs again. This tool is

the only way to go in today’s world.

Velocity of Propagation

A TDR needs to know the velocity of propaga-

tion(VOP) of the wire being tested before it

can begin.

VOP is measured as a percentage of the

speed of light (186,000 miles per second or

299,792,458 meters per second). A VOP

value of 100% simply means that a cable could

conduct electricity at the speed of light. But

because all cable has some inherent resistance,

the VOP value will always be less than 100%,

meaning that the electricity is running

through the cable at a percentage of the speed

of light. And as that percentage will vary with

size and type, the TDR must be calibrated for

the type of wire or cable you plan to test.

Although the AEMC Fault Mapper, the

tool I use, contains a fairly comprehensive

library of VOP values for commonly used net-

work and coaxial cabling, none of these come

close to the values for commonly used marine

wiring. Fortunately, this shortcoming is easily

overcome, as described below.

Fault Mapper CA7024, AEMC, http://www.aemc.com

Model 3271, Triplett, http://www.triplett.com

TIME DOMAIN REFLECTOMETERS

fault

Time Domain Reflectometry

transmitted

pulse

reflected

pulse

origin

Distance = t^1 + t 2 2 CO Vp

Co= speed of light in a vacuum

Vp= velocity of propagation

t 2

t 1

A TDR transmits pulses of a known shape and amplitude into one end of a
cable. The pulses travel along the cable at a speed determined by its VOP.
As they reach impedance changes in the insulation of the cable, indicating
a fault or a cable end, the reflections caused travel back along the cable
and are identified by the TDR. The size, shape, and general nature of the
reflected pulses indicate the type of fault encountered. The distance to the
fault can be determined from the time it takes for the pulse to be reflected.