Zone protective directional relay scheme

A protective system for protecting elements (for example D) of an electric power system allocates a protection zone to each element. The protection zones are defined by switching points (for example 19', . . . , 24') which isolate the associated element (for example D) from the power system. Each of these switching points is associated with a transmitter/actuator unit (for example 19, . . . , 24). The transmitter/actuator units (for example 19, . . . , 24) produce "open" commands for their associated switching points (for example 19', . . . , 24') if it is determined that a fault has occurred inside the associated protection zone. The "open" commands for the switching points (for example 19', . . . , 24') are formed locally within the vicinity of each of the switching points. This is realized by the transmitter/actuator units (for example 22) which contain two evaluating units (for example 22D, 22L8) which are respectively allocated to different protection zones, and in that all evaluating units allocated to one protection zone (for example 19D, . . . , 24D) communicate with each other via local data links (N) and remote data links (F).

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention is directed to a protective device for an electric power 
system. 
The invention relates to a system as described, for example, in 
Auslegeschrift No. 112,562 of the German Democratic Republic. In the known 
protective device for electric power systems, the individual objects to be 
protected can be isolated, in the event of a fault, via switching points 
constructed of circuit breakers. The faults occurring are in this case 
detected by fault-direction-oriented protection instruments. If a fault 
occurs within a protective zone which is allocated to the object to be 
protected and which is essentially determined by the switching points 
isolating the object to be protected from the power system, a centralised 
evaluating unit allocated to this protection zone provides switching-off 
commands to the switching points. 
The invention as characterised in the claims achieves the objective of 
creating a protective device of the generic type, in which device the 
switching-off commands to the switching points of a protection zone are in 
each case formed locally in the vicinity of each of these switching 
points. 
The protective device according to the invention is particularly 
characterised in that each switching point is associated with a 
transmitter/actuator unit, which transmitter/actuator units communicate 
with each other at the same hierarchical level. This provides 
decentralised operation of the protective device according to the 
invention, resulting in high availability and redundancy. 
The invention is explained below in greater detail with reference to the 
drawing, in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The energy distribution network of FIG. 1 shows two voltage levels. 
The first voltage level contains objects to be protected, such as busbars A 
and B, lines L.sub.1, L.sub.2, L.sub.3 and generators G.sub.1 and G.sub.3 
and the associated generator-transformers. This voltage level is connected 
to an adjacent power system N.sub.1. 
The second voltage level consists of busbars C.sub.1, C.sub.2, D, E.sub.1, 
E.sub.2 and F, lines and cables L.sub.4, L.sub.5, L.sub.6, L.sub.7, 
L.sub.8, L.sub.9, L.sub.10 and L.sub.11 and a generator G.sub.2 and its 
associated generator-transformer (without generator switch). This voltage 
level is connected to an adjacent power system N.sub.2. 
The voltage levels are linked by transformers T.sub.1, T.sub.2 and T.sub.3. 
In this arrangement, transformer T.sub.3 is a three-winding transformer. 
The tertiary winding of T.sub.3 is connected to a reactive-power 
compensator S (rotating or static). Another three-winding transformer 
T.sub.4 is connected to two further power systems N.sub.3 and N.sub.4. 
The lines L.sub.5 and L.sub.6 and L.sub.8 and L.sub.9 in each case form a 
double-circuit line and two systems having the same rated voltage on the 
same tower. Lines L.sub.3 and L.sub.10 are carried in parallel over the 
entire length of L.sub.10 on the same tower and form a double-circuit line 
having different rated voltages. 
The busbars C.sub.1 and C.sub.2 are constructed as dual busbars in which 
each branch, as for example transformers T.sub.1 and T.sub.2 and the lines 
L.sub.4, L.sub.5 and L.sub.6, can be optionally connected to each of the 
two busbars via isolating switches (shown as circles in the Figure). 
C.sub.1 and C.sub.2 are linked by a switching point 16' constructed as a 
tie switch. The busbars E.sub.1 and E.sub.2 form a 11/2 switch system in 
which one group each of two branches, such as L.sub.4 and T.sub.4 or 
L.sub.9 and G.sub.2 or L.sub.8 and L.sub.11 are connected via, in each 
case, three switching points 25', 28', 31' or 26', 29', 32' or 27', 30', 
33', constructed as circuit breakers, between busbars E.sub.1 and E.sub.2. 
This energy distribution system is divided into protection zones. Each of 
these protection zones is allocated to an object to be protected, for 
example to generator G.sub.1 or to busbar A. The boundaries of each 
protection zone are marked in FIG. 1 by thin and straight lines. The 
boundaries of the protection zones can be freely selected in accordance 
with the requirements of selectivity by a suitable arrangement of 
switching points 1', 2', . . . , 39', 40' (FIG. 2) on links between the 
individual objects of the energy distribution system or at its links to 
other system parts such as, for example, N.sub.1 or N.sub.2. The 
boundaries of the protection zones can be fixed or can be determined by 
the mimic diagram (isolator diagram) in accordance with current topology 
as, for example, in the case of protection of the busbars C.sub.1 and 
C.sub.2. Thus, for example, the isolator settings determine the boundaries 
of the protection zone, for example, in the case of busbars C.sub.1 and 
C.sub.2. In this arrangement, it is also possible that busbars are 
additionally further subdivided by sectionalising switches or ties. 
The protection zones can also be hierarchically nested so that inter-system 
faults can also be selectively detected. For example, the double-circuit 
lines L.sub.5 /L.sub.6 or L.sub.8 /L.sub.9 or L.sub.3 /L.sub.10 can be 
associated with higher-level protection zones which in each case comprise 
two protection zones of L.sub.5 and L.sub.6 or of L.sub.8 and L.sub.9 or 
L.sub.3 and L.sub.10 and provide the possibility of phase-selective 
disconnectionn of faults between two lines located on the same tower. 
A switching point is provided exactly at the transitions from one 
protection zone to another, for example the switching point 4' at the 
common boundary of the protection zones of line L.sub.1 and busbar A. The 
protection zone of line L.sub.1 is thus placed adjacent to the protection 
zone of busbar A. Each protection zone is limited by the switching points 
located in the links between the object located in the protection zone and 
the adjacent protection zones. For example, the protection zone allocated 
to busbar D is limited by switching points 19', 20', 21', 22', 23' and 
24'. 
Each switching point 1', 2', . . . , 40' is associated with a 
transmitter/actuator unit, not shown in FIG. 1, which primarily has the 
objective of determining fault-direction signals from current and voltage 
measurements at the switching point and, if necessary, providing a trip 
command to the associated switching point. The transmitter actuator units 
of switching points 1', . . . , 40' are essentially constructed in the 
same manner. 
In FIG. 2, the transmitter/actuator units 19, 20, 21, 22, 23, 24, 27 and 
30, which are allocated to the switching points 19', 20', 21', 22', 23', 
24', 27' and 30' determining the protection zones of the busbar D and of 
the line L.sub.8, respectively, are shown as representatives for these 
transmitter/actuator units. Switching point 22' forms the boundary between 
the protection zones mentioned. 
Each of the transmitter/actuator units 19, 20, 21, 22, 23, 24, 27 and 30 
shown in FIG. 2 contains measurement-value transmitters (represented by 
non-referenced transducer symbols) for current and voltage signals 
directly in front of switching point 19', 20', 21', 22', 23', 24', 27' and 
30' and an actuator (represented by an arrow) which acts on the associated 
switching point. In addition, each of the transmitter/actuator units 19; 
20; 21; 22; 23; 24; 27 and 30 successively contains in each case exactly 
two evaluating units 19L6, 19D; 20L5, 20D; 21L7, 21D; 22D, 22L8; 23D, 
23L9; 24D, 24L10; 27E1, 27L8 and 30L8, 30L11. These evaluating units are 
connected via local data links N, in continuous lines, and/or via remote 
data links F, in dashed lines, to all transmitter/actuator units allocated 
to the switching points of a protection zone. Thus, for example, the 
evaluating units 21D, 20D, 19D, 22D, 23D, 24D of the transmitter/actuator 
units 21, 20, 19, 22, 23, 24 of the switching points 21', 20', 19', 22', 
23', 24' allocated to busbar D are linked to each other via local data 
links N whereas the evaluating units 22L8 and 30L8 of the transmitter 
actuator units 22 and 30 of the switching points 22' and 30' allocated to 
line L8 are linked with each other via a remote data link F and only the 
evaluating units 27L8 and 30L8 of the transmitter/actuator units 27 and 30 
of the switching points 27' and 30' allocated to line L.sub.8 are linked 
with each other via a local data link N. 
In FIG. 3, the transmitter/actuator unit 22 is explained in greater detail 
unit 22 is representative of all transmitter/actuator units constructed in 
the same manner. Unit 22 contains a protective device DR (in dashed lines) 
having inputs which are connected phase by phase to the signal outputs of 
a current and a voltage transformer CT and VT, the two evaluating units 
22D and 22L8, a current sensing unit SU, a data link (symbolically 
designated as switch S') which can be activated and is placed between the 
two evaluating units 22D and 22L8, and data links V.sub.DR and V.sub.CB. 
In this arrangement, the measuring points of current and voltage 
tranformers CT and VT are connected in series with switching point 22' 
into the link between busbar D and line L.sub.8. These measuring points 
can also be interchanged with respect to the position drawn in FIG. 3. 
The boundary of the protection zone is established by the position of the 
current transformer CT. The path Z between the switching point 22' and the 
measuring point of the current transformer CT is called the "dead zone+. 
The protective device DR is a fault-direction detector. The fault-direction 
detector DR has the objective of detecting a fault, indicating the phase 
concerned and the direction of this fault with respect to the observation 
point, that is to say with respect to the measuring location (current and 
voltage transformer). For this purpose, it is necessary to measure current 
and voltage locally for each phase before and during the occurrence of the 
fault. Evaluation is carried out by suitably combining these measured 
values. In the fault-direction detector used in this case, step signals of 
current and voltage are formed by forming the difference, from which step 
signals the direction of the fault location in the phase concerned can be 
determined with respect to the measuring point with the aid of an 
arithmetic unit built into the fault-direction detector. The construction 
and operation of such fault-direction detectors are known, for example, 
from Swiss Auslegeschrift No. 5,642,491 and from European Patent 
Auslegeschrift No. 10,084,191. However, other valuation methods are also 
possible such as, for example, voltage-polarised overcurrent relays, 
relays with a combination of overcurrent and short-circuit power direction 
features or distance relays. 
To have a specific measurement orientation in the transmitter/actuator 
units, the sequence in which the switching point, for example 22', voltage 
transformer VT and current transformer CT are connected in series into the 
link between two objects, for example D and L.sub.8, are defined as the 
measurement orientation m (see FIG. 3). 
The local data link V.sub.DR provided in each of the transmitter/actuator 
units, for example 22, causes data to be moved from the fault-direction 
detector DR to the two evaluating units, for example 22D and 22L8, the 
transmitter/actuator unit concerned, for example 22, and the local data 
link V.sub.CB causes data to be moved from the two evaluating units, for 
example 22D and 22L8, and an actuating member, not shown, of the 
associated switching point, for example 22'. 
Each of the evaluating units, for example, 22D, is effectively connected 
via V.sub.DR to the protective device DR and, via the local data link N 
and/or remote data link F to the other transmitter/actuator units, for 
example 19, 20, 21, 23 and 24 of a protection zone, for example of busbar 
D. The effective connection to the other transmitter/actuator units, for 
example 19, can be established directly via a data link, for example, N, 
so that individual messages from the transmitter/actuator unit concerned, 
for example 19, are received, but it can also be established indirectly, 
for example via local data link N and/or remote data link F and at least 
one further transmitter/actuator unit, for example 19, 20 (see FIG. 2). 
The evaluating unit then receives from the adjacently arranged 
transmitter/actuator unit, for example 19, group messages which contain 
information items on the state of transmitter actuator units, for example 
20, 21 which are effectively connected to the transmitter EM only via the 
adjacently arranged transmitter/actuator unit, for example 19. Each of the 
evaluating units, for example 22D is effectively connected via V.sub.CB to 
the switching point, for example 22', which is allocated to its 
transmitter/actuator unit, for example 22, and directly or indirectly via 
the local data link N and/or remote data links F to the other 
transmitter/actuator units, for example 19, 20, 21, 23 and 24 of a 
protection zone, for example of the busbar D. 
The following binary signals can be transmitted via the local data links 
V.sub.CB and V.sub.DR : 
Switching point status (open/closed, ready for operation not ready for 
operation (diagnostic)), 
Status of protective devices DR (functional/non-functional (self-diagnosis 
in progress)), 
Report as to whether voltage is applied to the measuring point, 
Direction decision of the protective device DR constructed as 
fault-direction detector: fault in orientation of measurement, opposite to 
orientation of measurement, undecided, 
Opening command to switching point, 
Execution report of switching point opening (possibly supplemented by 
back-up switch protection), 
Messages sent by a current-sensing unit SU whether the current at the 
measuring point has dropped to zero (if switching point open and current 
not equal to zero at the measuring point, a unilaterally supplied fault is 
present in the "dead zone"), 
Switching-on commands to the switching point (reconnection). 
FIG. 4 shows a communications network of a protective device according to 
the invention. This communications network displays largely the same 
topological configuration as the energy distribution system to be 
protected which is shown in FIG. 1. The transmitter/actuator units are 
designated by reference FIGS. 1, 2 . . . , 39, 40. Each 
transmitter/actuator unit, for example, 3, are associated with exactly two 
evaluating units, such as, for example, 3G1, 3A. Allocation is carried out 
via local data links between all evaluating units of all transmitter 
actuator units which are arranged at the boundaries of the protection zone 
of an object to be protected. If the object to be protected is, for 
example, busbar A, the associated evaluating units 3A, 4A, 7A and 8A are 
linked with each other by local data links. According to FIG. 2, the 
relevant local data links are shown as continuous lines also in FIG. 4. 
In the case of busbar A, the transmitter/actuator units 3, 4, 7 and 8 are 
located close together in a substation. However, it is also conceivable 
that the protection zone is geographically dispersed as in the case of a 
line, a cable or with extended switching systems (distance between the 
transmitter/actuator units greater than several hundred meters). At each 
of the ends of such an object to be protected, which can be an overhead 
line, a cable or an extended switching system, for example L.sub.3, 
L.sub.8, L.sub.9 or L.sub.10, one evaluating unit, for example 10L3 and 
36L3; 22L8, 27L8 and 30L8; 23L9, 26L9 and 29L9 or 24L10 and 35L10 are 
arranged. The evaluating units of each object cooperate via remote data 
links. According to FIG. 2, the subject remote data links F are shown in 
dashed lines. See also FIG. 4. 
In cases in which faults can occur between two objects to be protected, for 
example between lines L.sub.5 and L.sub.6, L.sub.8 and L.sub.9 or L.sub.10 
and L.sub.3, the associated evaluating units must be matched to each 
other. This is possible by additional data links generally constructed as 
local links. Such links are provided, for example, between 15L5 and 18L6, 
20L5 and 19L6, 22L8 and 23L9, 27L8 and 26L9, 35L10 and 36L3 and 24L10 and 
10L3. 
The local data links N can be optionally constructed as a bus system or 
permanently installed conductors (point to point links). In FIG. 4, for 
example, the evaluating units 4A, 3A, 7A and 8A are linked with each other 
by means of a bus system but the evaluating units 5B, 6B, 9B and 10B, in 
contrast, are linked with each other by point to point links. If the 
requirements on the redundancy of the data links are lower, individual 
links between 5B, 6B, 9B and 10B can also be omitted, leaving a tree 
structure, or can be activated only if tree branches fail. 
The remote data links F can be, for example, optical waveguides 
(polytetrafluoroethylene), cable cores, directional radio or 
carrier-frequency links. 
It is sufficient if at least one unsecured (fast) transmission link is 
available since then a directional criterion determined by the protective 
device DR can be transmitted in case of a fault. But if back-up functions 
are also to be provided, a secured (slow) link must also be provided for 
transmitting a direct or indirect switching-off command. 
The local data links N and remote data links F transmit the following 
signals: 
(1) All transmitter/actuator units involved indicate a fault direction to 
the object to be protected (global message) or at least one 
transmitter/actuator unit involved indicates fault direction from the 
object to be protected or is undecided, faulty and so forth 
and/or 
a specified group within the transmitter/actuator units involved indicates 
a fault direction to the object to be protected (group message) or at 
least one transmitter/actuator unit involved from this specified group 
indicates fault direction from the object to be protected or is undecided, 
faulty and so forth. The specified group can be different for each data 
link within the protection zone. The specified group can also comprise 
only one transmitter/actuator unit (single message). 
(2) The signals described in the preceding paragraph (1) can also be 
represented in complementary or other forms in accordance with Boolean 
algebra rules. 
(3) Dead-zone detection signals 
(4) Back-up switch signals 
(5) All signals transmitted by data links V.sub.DR and V.sub.CB can also be 
transmitted by local data links N and remote data links F either 
individually or in a suitably preprocessed form. 
The signals specified in the above list are of a binary character. 
The protective device according to the invention operates as follows: 
Each evaluating unit has the objective of detecting faults and identifying 
the location of faults inside or outside the protection zone of one of the 
objects on the basis of the information of the protective device DR of the 
associated transmitter/actuator unit and the incoming information from the 
remaining transmitter/actuator units of a protection zone and, if 
necessary, tripping its respective switching point. 
In this context, means are suitably provided in each evaluating unit for 
blocking the evaluating unit in the case of switching actions within the 
protection zone since it must be assumed that the protective device DR may 
not possibly be able to differentiate between signals caused by faults and 
signals caused by switching actions. 
The evaluating unit, for example 22D, receives the signals arising from the 
associated protection device DR and the remaining evaluating units, for 
example 19D, 20D, 21D, 23D, 24D associated with a protection zone. By way 
of example busbar D combines these signals (normal operation of the 
transmitter/actuator unit with a data flow marked by directional arrows in 
FIG. 3). If the fault has been recognized as being located inside the 
common protection zone after evaluation of all direction signals of the 
protective devices DR, taking into account the measurement orientation m 
of the transmitter/actuator units, a signal is locally emitted which opens 
its own switching point, for example 22'. The same is true for evaluating 
units 19D, 20D, 21D, 23D and 24D with respect to switching points 19', 
20', 21', 23' and 24'. In the tripping case, each current-sensing unit SU 
monitors whether the currents of current transformers CT of the associated 
measuring point have become zero after an expected period associated with 
switching points, for example 22', 24' of the transmitter/actuator units, 
for example 22, 24, has elapsed. This is reliably the case if the fault is 
located inside the protection zone concerned, neglecting a "dead zone" 
between the current transformer of the measuring point and the switching 
point. 
As can be seen from FIG. 3, at each of the transmitter/actuator units, for 
example, 22, a "dead zone" is located between current transformer CT of 
the measuring point and the switching point, for example 22'. As a result, 
in case of a fault, the direction relay DR of the subject 
transmitter/actuator unit recognizes the fault, shown by a zig-zag arrow, 
identifying it as for example an earth leak or a short circuit, and as 
being located opposite to the measurement orientation m. Assume that 
transmitter/actuator unit 22 is arranged, for example, in such a manner 
that its associated switching point 22' is connected to busbar D and its 
associated measuring point is connected to line L.sub.8 (direction arrow m 
of the transmitter/actuator unit 22 in FIG. 3). If a fault occurs in the 
dead zone located between the switching point and the measuring point of 
this unit, the evaluating units 19D, 20D, 21D, 22D, 23D, 24D will 
recognize this fault as being located inside the protection zone delimited 
by switching points 19', 20', 21', 22', 23' and 24' and evaluating units 
22L8, 27L8 and 30L8 will recognize it as being located outside the 
protection zone delimited by switching points 22', 27', 30'. Evaluation 
units 19D, 20D, 21D, 22D, 23D and 24D cause the switching points 19' to 
24' associated with the transmitter/actuator units 19 to 24 to open and 
busbar D is isolated even though the remaining system is still faulty in 
line L.sub.8. 
The current-sensing unit SU provided in transmitter actuator unit 22, 
however, detects that the fault current in the transmitter/actuator unit 
22 has not disappeared despite the open switching point 22'. It then forms 
a dead zone detection signal t.sub.1 which acts via a data link 
V.sub.R,t1, evaluating unit 22D and local data links N to 23D, 19D etc. on 
the remaining transmitter/actuator units 19, 20, 21, 23 and 24 in which 
the fault current has disappeared. As a result, the above mentioned 
transmitter/actuator units reclose their associated switching points 19', 
20', 21', 23' and 24'. Thus, the current-sensing unit SU in 22 also 
detects that the fault is in the "dead zone" of its own 
transmitter/actuator unit and forms another dead-zone detection signal 
t.sub.2 which, via a data link V.sub.R,t.sub.2, evaluating unit 22L8 and 
remote data link F and local data link N, causes evaluating units 27L8 and 
30L8 to open switching points 27' and 30' associated with 
transmitter/actuator unit 27 and 30. As a result, line L.sub.8 connected 
to the faulty "dead zone" is isolated. 
It is conceivable that a fault occurs in the "dead zone" when the switching 
point, for example 22' of a transmitter/actuator unit, for example 22, is 
open (see FIG. 3). The pre-fault condition under which such a fault can 
occur is characterized by three signals designated "switching point open", 
"measuring point under voltage" and "current at the measuring point equals 
zero". These signals are evaluated in two evaluating units, for example 
22D and 22L8 of the associated transmitter/actuator unit, for example 22. 
In the evaluating unit of the protection zone located in the direction of 
the measurement orientation m of the transmitter/actuator unit, for 
example 22, for example that of line L.sub.8, the direction signal of DR 
is then evaluated in the opposite sense (reversal of direction) and this 
direction signal is suppressed (blocked) in the other evaluating unit, for 
example 22D. 
If then, when a fault occurs in the "dead zone" for instance of 
transmitter/actuator unit 22, the associated switching point 22' is 
already opened, the fault direction detector DR associated with the 
transmitter/actuator unit 22 reports that a fault has occurred in the 
protection zone allocated to busbar D whereas the fault direction 
detectors contained in the transmitter/actuator units 19, 20, 21, 23 and 
24 do not specify any fault direction at all since, of course, the 
switching point 22' located between the busbar D and, therefore, 
transmitter/actuator units 19, 20, 21, 23 and 24 and the fault location is 
open. 
If the direction signal from protective device DR in the evaluating unit 
22D is blocked, units 19D and 24D are prevented from responding. An 
inversion of the same direction signal in evaluating unit 22L8 causes the 
fault to be recognized as being located in the protection zone of line 
L.sub.8 and evaluating units 27L8 and 30L8 provide tripping signals to 
their switching points 27 and 30 which causes line L.sub.8 connected to 
the faulty "dead zone" to be isolated. 
The protective device according to the invention also makes it possible to 
implement back-up switch protection. If a switching point associated with 
a transmitter actuator unit fails, the current-sensing unit SU of this 
transmitter/actuator unit detects, in the case of a fault, that the 
current does not drop to zero in the transmitter actuator unit although 
the transmitter/actuator unit has formed a command for opening the 
switching point. 
In relation to switching point 22' if, for example, a fault is present on 
line L.sub.8, the fault is detected by transmitter/actuator units 22, 27 
and 30 and evaluated in the associated evaluating units 22L8, 27L8 and 
30L8. Evaluating units 27L8 and 30L8 supply the tripping commands for 
opening switching points 27' and 30' to 27 and 30, respectively. Unit 22L8 
provides 22 with the tripping command for opening switching point 22'. 
However, switching point 22' may not open and the current in current 
transformer CT of 22 may not drop to zero after the natural period of the 
switch (independently of the acknowledgement by auxiliary contacts of the 
switch of the switching point). 
This switch failure is detected by SU in 22. SU supplies via the local data 
links V.sub.R,t.sub.1 and V.sub.R,t.sub.2 a back-up switch signal R to the 
evaluating units 22D and 22L8. 22D passes the back-up switch signal R via 
the local data link N to 19, 20 21, 23, 24, which causes these switches to 
trip. 
If, in the case of a defective switch of the transmitter/actuator unit 22, 
the fault is located on the busbar D, the evaluating units 19D to 24D 
supply tripping commands to 19 to 24. As described before, 22 fails and 
reports its failure to 22D and 22L8. 22L8, corresponding to 22D, emits a 
back-up switch signal. 22L8 and 30L8 then trip switch positions 27' and 
30'. 
The protective device according to the invention can be simply constructed 
in such a manner that even a failure of the protective device DR and of 
current-sensing unit SU of a transmitter/actuator unit will not impair the 
protective function. To this end, in each of the transmitter actuator 
units, for example 22, a self-monitoring unit SU, shown in FIG. 3, is 
provided which continuously monitors the operational integrity of DR and 
SU and reports any failures to the two associated evaluating units, for 
example 22D and 22L8. If DR and/or SU are non-operational, a data link 
located between the two evaluating units, for example 22D and 22L8 is 
activated (switch symbol S' in FIG. 3) which connects the two evaluating 
units together as a result of which the protection zones of the two 
relevant objects to be protected, for example D and L.sub.8, form a common 
protection zone. As a result of the protection zones being connected 
together in this manner, the protective devices PU do not need to be 
duplicated. 
The operational integrity of the evaluating units, for example 22D, 22L8 
within a transmitter/actuator unit can be ensured through redundancy 
arrangements (duplication or 2-of-3 arrangement). 
To keep the protective device according to the invention fully functional 
even in the case of a failure of the local data link N and/or remote data 
link F, these links are suitably arranged in a redundant network. Such a 
redundant network would then have the characteristic that, if one of the 
links is interrupted, the signal to be transmitted reaches its destination 
via a redundant back-up link. If, for example in the communications 
network according to FIG. 4, the remote data link from 17L4 to 25L4 and 
28L4 were interrupted, the information flow could be maintained via a 
back-up line which leads from 17L4 via the remote links between 15L5 and 
20L5 and 23L9 and 29L9 to 25L4 and 28L4.