ML19319B492

Facility & Instrument Grounding Sys, Revision 1

ML19319B492
Person / Time
Site:	Davis Besse
Issue date:	08/30/1977
From:	TOLEDO EDISON CO.
To:
Shared Package
ML19319B490	List: ML19319B489 ML19319B492
References
TAC-10996, NUDOCS 8001220900
Download: ML19319B492 (4)

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Attachm:nt to TECo let'sr Serial No. 387 dated September 16, 15 7 Revision 1 to Attachment to TECo letter

{Jx_) Serial No. 3J6 dated August 30, 1977 DAVIS-BESSE UNIT 1 ST TION AND INSTRUMENT GRol3 DING SYSTEMS SYSTEM DESIGN CRITERIA The station ground grid system at Davis-Besse is a three dimen-sional grid in the turbine and auxiliary building that consists of 4/0 and 500 MCM bare copper conductor. The vertical risers are in-stalled at a minimun of every other building column. Horizontal conductors are installed as needed to tie equipment to the station ground grid (20' to 70' horizontally). The station ground grid vertical risers are tied to the building steel at each floor as a minimum and the horizontal conductors are tied to the building steel at every other building column as a minimum. Drawing i *70, Rev. A shows some typical equipment that is tied to the station ground grid. This drawing was left with the NRC during our meeting on August 25, 1977 and a copy is attached.

The instrument grounding syctem has been installed as shown on drawing E-470, Rev. A. The inctrumentation common sianal and analog signal cable shields for the following major systems are tied to the instrument ground bus:

1. Reactor Protection System (RPS).

2. Integrated Control System (ICS).

3. Non-Nuclear Instrumentation (NNI).

4. Computer processing unit, local and remote multiplexers, typers, line printer, paper tape punch / reader, card reader, cathode ray tubes and operator keyboards.

5. Miscellaneous electronic control system.

The analog signal cable shields for the following major systems are tied to the instrument grcund bus:

1. Safety Features Acutation System (SFAS)

2. Steam and Feedwater Rupture Control System (SFRCS)

3. Steam generator level 8001220 fo o Revision 1

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g s t i i The station ano instrument grounding systems nave been installed in

1 j accordance with the Davis-Besse Unit I design criteria and as shown t on drawing E-470, Rev. A. During the testing on these systems, the l following has been found:

l j 1. RPS, ICS and NNI; the instrument ground buses in these l

systems are intentionally tied together as shown on

drawing E-470, Rev. A. There may be an inadvertent tie between this group of instrument ground buses and either the station ground system or the instrument ground bus

.I in the ccmputer or the miscellaneous electronic control system.

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! 2. There is an inadvertent tie between the following in- ,

strument ground bus and either the station ground system

! or the instrument ground bus in another system:

I A. Computer. ,

i B. Miscellaneous electronic control system.

l EQUIP:1ENT DESIGN CRITERIA The RPS was designed and shipped to Davis-Besse with isolated in- ,

' strument and station ground systems. The RPS has been used at all B&W units in operation with these two buses tied together in the cabinets and then connected to the station ground gri t. The RPS is I designed to operate within specifications when installed with a j single grot.nc tns; system or with separate station and instrument grounds as stecified at Davis-Besse. The Davis-Besse grounding system meets the B&W balance-of plant (BOP) requirements for this installation. The E&W E0P requirements stated in the 205 BSAR call for a grounding system similar to that employed at Davis-Besse.

4 Th n NNI and ICS were designed to have either single or separate in-st:ument and s;ation ground systems. All B&W units in operation hare had the two ground buses in these systems tied together. The NNI and ICS caoinets and interconnecting cables to remate instruments were shipped to Davis-Besse with the two ground systems tied together.

This required Toledo Edison to work closely with B&W and Bailey field representatives to modify the~ equipment to separate the j ground buses in these systems.

l The computer and miscellaneous electronic control system were designed and shipped with separate instrument and station ground

, systems.

TheShASandSERCSandsteamgeneratorlevelsystemsweredesigned and shipped with separate instrument and station ground systems.

Only analog signal cable shields are connected to the instrument 4 ground bus. Noise tests conducted by their manufacturer (Consolidated i i Controls Corporation) indicated that these shields could be connected i to either the station or instrument ground system or they could be j

1 cit ungrounded.

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MAXIMUM GROUND FAULT CURRENTS j If a ground fault on any of the major electrical equipment, station 1 power electrical buses or transformers should occur, this fault j current would flow through the station ground system. The maximum ground fault current that could occur due to a fault on each elec-l trical system on Davis-Besse Unit 1 is as follows:

1. 345 KV - 21,200 amps.

2. 25 KV Main Generator - 8 amps. Limited by ground 4 resistor.

3. 13.8 KV Housepower buses - 400 amps. Limited by ground resistor.

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4. 4.16 KV Housepower buses - 400 amps. Limited by ground resistor.

5. 480 Volt Housepower buses - 21,900 amps.

j 6. 250/125 Volt DC Housepower buses are ungrounded and no j DC fault currents can flow in the station ground grid.

MAXIMLH GROUND CURRENT INDUCED VOLTAGES i

I If there is an inadvertent tie between the instrument ground system in the RPS and the station ground, loop fault current from an elcc-trical !ault could flow in the RPS instrument ground connection producing an induced voltage at the RPS system.

The largest ground currents that can flow through the wAcion i

ground system in the Cable Spreading Room ceiling would be caused i by a ground fault in the 480 V Motor Control Center (MCC) located directly above the Control / Cabinet Room at elevation 643'. The calculated ground fault current that can occur at this MCC is 4,224 I amps. The source of this ground fault current is a 1000 KVA transformer located on elevation 603' at the northside of the auxiliary building.

j This 1000 KVA transformer has a 6.75% impedance on 1 MVA base, 13.8 i KVA to 480 V, (Delta : Y) transformer with a solidly grounded neutral.

! The 13.8 KVA bus is assumed to have zero impedance up to the 480 volt transformer.

It was found that approximately 90% of the ground fault current 1 would flow from the MCC to the 1000 KVA transformer on ene ground cables installed in the conduits with the phase cables.

In calculating this induced voltage that could be impressed on the RPS instrument ground bus, the following conservative assumptions were l made:

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1. ,The fault current at the MCC enters the station ground grid and flows

. through three paths to the source transformer neutral. ,

2. No ground fault current would be by-passed through building steel. lI

! 3. The location of the inadvertent tie between the instrument and I station ground systems was picked in an attempt to maximize the loop currents flowing in the RPS instrument ground buses.

4. The fault current flowing in the instrument ground bus of the RPS cabinet with the inadvertent ground returns to the source transformer across the impedance of the 500 kcm insulated instrument bus grounding cable.

l j Using the above conservative assumptions, the f ault voltage that could be impressed on the instrument ground buses would be 1.4 volts, rms, 60 Hz.

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The ground fault current that caused this voltage would be interrupted within l1 3 cycle. (50 ms.). This 1.4 volt difference between NI/RPS cabinets would i

cause 60 He current flow through the system's instrument ground. Normally, this kind of " noise" could enter the system through inductive or capacitive coupling between a ground lead carrying the noise and an analog signal lead.

I Since the degree of coupling is not known, one can assume an unlikely worst case in which there is direct coupling, with no attenuation, directly into any or all RPS 0 to 10 volt DC analog signals. The system is designed to reject such noise by a minimum of 34 db which translater to a 2500:1 power attenuation or a 50:1 voltage attentuation. With 34 db rejection, the worst effect on a trip set point will be to impose an error of less than 0.24% of the signal full range. However, when the noise is injected in such a way as to pass through

one or more modules prior to be bistable set point module, additional attenua-i tion occurs reducing the error to less than 0.06%. Considering the assumed direct coupling, the effect is extremely conservative. It is B&W's accessment that this is not a safety concern.

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