Pilot Protection Terminals
15kV Insulation Level
PPT/35
The pilot protection terminal type PPT/35 has been designed
and developed by Weidmüller to meet the ESI standard
12-4 1973. ‘Terminating Equipment for pilot cables subject
to induced transient voltages exceeding 650V rms’. The
test terminal, see fig 1, consists of a longitudinal disconnect
terminal with plug bolts on both sides of the disconnection
into which earth conductors can be plugged prior to
removal of the disconnect plug. Removal of this
plug can only be effected by the use
of a special handle. The creepage
and clearance paths between
terminals, between the
terminal and earth and
across the disconnect have
been designed to give
an insulation test level of
15kV a.c. for 1 minute.
The clamping yokes are
type ‘B’ to EA Technical
Specification 50-18 i.e.,
screw clamp/spring
loaded insertion type, to
accommodate hook bladed
crimps to EA Technical
Specification 50-18.
The terminal is designed for
mounting onto plastic rail type
TSK35 dimensioned according
to EN 50 022 (BS.5584:1978).
Fig. 1
Terminating Box
Weidmüller are able to offer the pilot terminating box as a complete unit for testing, disconnecting and
earthing the pilot cables.
Fig 2 shows a typical layout in a TB14 box.
Other standard box layouts are available.
It comprises PPT/35 terminals mounted on TSK35 insulated mounting rail together with the necessary
earth bar connections and interconnecting leads, insulated gland plate, pilot cable armour disconnect
link and removable transparent covers to protect against accidental touching during normal operation.
The test leads and disconnecting plug handles are housed in the door of the enclosure.
Transmission system pilot cables
Transmission systems for the bulk transfer of electrical energy are typically comprised of one or
more three-phase power circuits operating at supertension i.e., voltages in excess of 33kV, which
interconnect substations where the supply is boosted via a transformer from a generating station,
or transformed down to lower voltages for distribution and utilisation. The transmission medium is
either underground power cables or overhead power lines. In either case, supervision and control
of the system is effected via low voltage auxiliary cables (or pilots) laid along the same physical length.
From the inception of these concepts in Britain in the fifties, it was realised that transient disturbances,
which could be a hazard to personnel, could occur in the pilot cables under an earth-fault condition
on any power line.
Under such a system earth-fault, a proportion of the earth-fault current will return to the earth star-
points of the transformers at the substation site. The current which flows to that site through the
ground will cause the site and all ‘earthed’ metal therein to rise in potential with respect to true earth.
This voltage rise would be impressed across anyone on the site if he were working on pilot cables or
equipment directly connected thereto, even though the pilots are not earthed. It is very important to
note that such a rise-in-earth potential could be caused by faults on other power circuits, in addition
to those directly associated with the pilot which is being worked on.
In underground power cable systems, the associated pilot cables are almost invariably laid alongside
the power cables, whereas for overhead line systems any associated pilot cables may
be laid in the vicinity of the overhead lines. Under power-system earth fault
conditions, the fault current flowing through the power
circuits can induce high end-to-end
voltages (of several kV) in
adjacent pilot cables.
Fig. 2
Protection against
induced voltages
Measures to avoid the risk of shock
from pilots or equipment connected
thereto include:
q
Disconnection of the equipment from
the pilots by removing appropriate links.
q
Earthing of the pilot circuits at a
convenient position near the point of
work.
q
Connection of metering, control and
telecommunication circuits via barrier
equipment.
The terminating equipment and barrier equipment are normally located in
separate housings as shown in this diagram. The barrier equipment comprises
isolating transformers which give the required insulation level.
Fig. 3: Single line diagram of pilot cable terminating equipment
Electricity Supply
Industry Standard
The Electricity Supply Industry
Standard ESI 12-4 1973 gives a
detailed specification for pilot
termination and barrier equipment.
The main functional requirements of
this equipment are:
q
Insulation of pilot circuits from earth
and from station wiring to the specified
induced voltage level, arranged so as
to eliminate the possibility of casual
contact by personnel.
q
Disconnection of pilot cable conductors
from local pilot circuits.
q
Application of test connections to pilot
cable conductors and/or local pilot
circuits.
q
Earthing of pilot cable conductors
and/or local pilot circuits.
q
Connection of all pilot cable conductors
together, but free from earth.
q
Due to its position at the end of the
pilot cable, the terminating box is also
the point where the metallic covering of
the pilot cable, normally earthed, can
be disconnected from earth so as to
apply a voltage test to check the
integrity of the cable oversheath. An
insulated gland-plate is therefore
necessary.
Terminating Equipment
Barrier Equipment
This is normally built into other
switchgear control panels located in
the substation control buildings and
hence terminals from the standard
Weidmüller range are often used.
It should be noted that on the local
pilot circuit side of the isolating
transformers, the terminals (item A of
Fig 4) are still subject to the transient
high voltage surges under system
earth-fault conditions. If Weidmüller
terminals are used here, or as feed
through terminals to some other piece
of protected equipment, the 15kV
insulation level must be maintained,
and to achieve this we recommend
that Weidmüller terminal type SAKH6
should be used.
Weidmüller can quote for special
assemblies of equipment in
enclosures for this or any other
type of protection on provision of
full specification of requirements.
References
Engineering Recommendation S.5/1
‘Earthing Installations in Substations’
E.R.A. Report M/T 126, 1958
‘Interference between power systems
& telecommunication lines’
EA Tech Spec 50-18:
‘Design & Application of Ancillary
Electrical Equipment’ including
‘Terminal Blocks’ and ‘Electrical
Terminations’
EA Tech Spec 09-6:
‘Auxiliary Multicore & Multipair Cables’
ESI 12-4:
‘Terminating Equipment for Pilot
Cables subject to Induced Transient
Voltages exceeding 650V rms’
EN 50 022:
(BS 5584:1978) ‘Low Voltage
Switchgear and Controlgear for
Industrial Use. Mounting rails 35mm
wide for snap-on mounting of
equipment.’
Fig. 4: Typical arrangement of terminating equipment
Technical Specification
PPT/35
Ordering Data
Type
PPT/35
Cat No
383502
383512
383526
383536
051430
382706
029960
105900
one plug
SAKH6
Ordering Data
Type
SAKH6
AP
TS32
SST3
EWK1
Dekafix 5
Cat No
012660
013170
012280
015270
020616
Terminal (complete with
plug but less handle)
End Plate
AP/PPT
Plug Handle
PH/PPT
Plug (Spare)
TST/PPT
Assembly Rail (Insulated)
TSK35
Locking Pin
SST1.6
Test Plug
PS
End Bracket
WEW35
Marking Tags
Dekafix 5
N.B. Standard pack terminal (
383502
) contains
handle (
383526
)
USE OF HANDLE IS MANDATORY
Terminal
End Plate
Mounting Rail
Locking Pin
End Bracket
Marking Tags
General Data
Terminal Thickness
15mm
Material:
Terminal Body . . . . . . . . Melamine
Disconnect Plug . . . . . . . Polyamide
Plug Handle . . . . . . . . . . Polyamide
Insulation Stripping Length
12mm
Conductor Sizes:
Solid . . . . . . . . . . . . . . . 0.5 - 10mm2
Stranded . . . . . . . . . . . . 0.5 - 10mm2
. . . . . . . . . . . . . . . . . . . or 2 hook blade crimps
Terminal Thickness
Material
14mm
Epoxy
Insulation Stripping Length
Conductor Sizes:
12mm
Solid . . . . . . . . . . . . . . . 0.5 - 6mm2
Stranded . . . . . . . . . . . . 0.5 - 6mm2
Ratings
Voltage (for one minute)
Current
15kV a.c. (ESI 12-4)*
30 Amps (IEC 947-7-1)
Rated Voltage
Flashover voltage
Current
* Mounted on plastic rail
1000 Vac
(IEC 947-7-1)
1500 Vdc
Terminal to terminal >15kV
47 Amps
(IEC 947-7-1)
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