DT3000
Instruction Leaflet
I.B. 17555D
Effective: Date 07/02
Page 75
For more information visit:
www.cutler-hammer.eaton.com
Supersedes I.B. 17555C dated November 1999
The Digitrip 3000 is a true rms measuring device, and will
integrate the spikes and dead periods to arrive at a current
measurement. The user should follow standard application
guidelines of comparing the Ct saturation curve with the
total connected burden, in light of the maximum fault
current. The total burden includes all connected measuring
device current windings, plus resistance of the Ct
secondary winding itself and all interconnecting wiring.
Since saturation curves are plotted with rms-measuring
instruments, measurement errors and tripping times can
be predicted with good reliability.
The Digitrip 3000 with DSPS can help to reduce Ct
saturation problems. The current-powering transformers
have been designed to present lower burden than most
“self-powered” relays, especially for large fault current
magnitudes. The burden is much lower than that of an
electromechanical relay.
B6.4 BURDEN DATA
In normal operating conditions, the burden is <0.08 ohms
with 3-phase 1A Ct current, or 0.2 per unit, and drops to
<0.04 ohms at high current levels. Figures B-6 and B-7
present Ct burden data in ohms and volt-amperes. In these
cases, the burden shown is the total Ct terminal value,
which is the DSPS plus the relay measuring circuits, for the
indicated operating condition.
Figure B-6 shows burden impedance magnitude in ohms.
The two lower curves are the values with ac power applied;
the upper two are with Ct powering only. For each of these
pairs, one curve shows the burden for a single-phase
current (representing a single-phase-to-ground fault) and
the other for three balanced phases with normally arrayed
120-degree phase angle increments. There is no phase
sequence sensitivity.
Figure B-7 shows the burden in volt-amperes for the same
four cases.
The three-phase burden cases assume the normal angular
distribution of the phases at 120-degree intervals. If the
three-phase current inputs are connected in series to a
single current source for a lab-bench test, burden results
will be slightly different.
B7.0 TESTING THE DUAL-SOURCE POWER SUPPLY
The DSPS requires no maintenance or adjustment. Use
the following procedures to check for proper operation.
First, confirm or test Digitrip 3000 operation with ac power
applied, according to the guidelines of the DT3000. Then,
the following can be used to confirm that the DSPS is
functioning correctly.
B7.1 IN-SERVICE TEST
THERE MAY BE LIVE VOLTAGES AND
CURRENTS PRESENT ON AND AROUND THE
DIGITRIP 3000. ONLY QUALIFIED PERSONS
SHOULD BE PERMITTED TO PERFORM ANY
WORK ASSOCIATED WITH THE EQUIPMENT.
FAILURE TO FOLLOW SAFE PRACTICE COULD
RESULT IN EQUIPMENT DAMAGE, SERIOUS
INJURY AND/OR DEATH..
•
Some visible load current should be flowing in the
protected feeder circuit. It does not need to be full
rated load. Check all three phases.
•
Consider disconnecting trip circuits during this live
testing, although no tripping tests are called for in the
following.
•
Connect a dc voltmeter across Digitrip 3000 terminals
5 & 6 on TB1. This is on the left side of the relay as
viewed from the rear.
•
With ac power applied, this dc meter should read
approximately 50 volts.
•
Connect an isolated ac multi-meter across any one
phase Ct input and observe the voltage drop, which
will be less — maybe much less — than 0.25 volt. See
Figure B-8.
•
Disconnect ac power from the relay. The front panel
and relay data communications may go away if the
load current is small.
•
The ac voltage drop across the Ct input should rise to
a larger value, at least twice as large as when ac was
applied, and maybe approaching 0.5 volt as shown in
Figure B-8.
•
The dc voltage on terminals TB1 - 5 & 6 should
correspond very roughly to that from the plot in Figure
B-9, DSPS output voltage as a function of three-phase
current.
•
Disconnect instruments, restore ac power to relay,
reconnect trip circuits, and check for resumption of
front panel & communications operation.
B7.2 LAB BENCH TEST
1. With ac control power applied, set the relay to trip well
below 9 A — any phase trip function is OK.
2. Apply more than 9 A to Phase A and confirm tripping.
Remove current and reset relay.
3. Remove ac control voltage — relay turns off.
4. Apply 9 A to Phase A again. Relay should power up
and trip. No need to recheck timing, but it may be
observed to be about 100 ms longer than for ac
control powered case. If test current is well above 10
A, avoid applying it for a long period.
5. Repeat step 3 for Phase B.
6. Repeat steps 3 for Phase C, however, this time, use
an isolated ac multi-meter to measure the voltage
