ML19319C436
| ML19319C436 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 09/13/1977 |
| From: | Engle L Office of Nuclear Reactor Regulation |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| TAC-10996, NUDOCS 8002140816 | |
| Download: ML19319C436 (9) | |
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SEP 13 BI7 DOCKET N0. 50-346 APPLICANT: TOLED0 EDIS0N COMPANY FACILITY:
DAVIS-BESSE, UNIT 1 SACKGROUND AND SU. ARY OF !EETING HELD ON AUGUST 25, 1977 WITH T
SUBJECT:
THE TOLED0 EDISON CDMPANY PIGARDING THE INSTRUPINT-STATION GROUND GRID SYSTEM FOR THE DAVIS-BESSE, UNIT 1 A meeting was held on August 25, 1977 with the Toledo Edison Company and the HRC staff regarding the presently installed instrurent to station ground gridsystem at Davis-Besse, Unit 1.
An attendance list is orovided in.
Background
On August 17, 1977 the Toledo Edison Company at a meeting with the NRC staff reviewed the design criteria for the Davis-Besse, Unit 1 instrument to station ground grid system. As a result of station testing it had been determined that there may be inadvertent ties between the instrument and station ground at other than the commonly designed tie point. The systems that may have the station ground and instrument ground connected included the Reactor Protection Systen-Nuclear Instrurentation, Non-Nuclear Instrts.entation, Station Conputer, Integrated Control System, and the non-essential Miscellareous Electric Control System.
Summary The Toledo Ediscn Company presented their analysis of the potential effect of these possible inadvertent ties on the safety instrtmentation systems for a postulated worst possible electrical fault that would introduce ground currents into the station electrical ground grid system. A su:: nary of their analysis presented at the meeting is provided in Enclosure 2.
Based on the analysis for worst case faulted conditions, the Toledo Edison Company stated that safety systems will perform their intended function under the worst postulated station electrical fault condition.
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- e r3 SEP 131977 At the close of the meeting, the NRC staff infonned the Toledo Edison Company that they would expedite their review and evaluation of the infonnation presented and be in further contact with the Toledo Edison Company on the following day regarding these matters, d,,,;,u Or!*al signed bz h tg
% John F. Stols Leon Engle, Project Manager Light Water Reactors Branch No.1 Division of Project Management
Enclosures:
1.
Attendance List 2.
Analysis-Davis-Besse, Unit 1 Station and Instrument Grounding System ces w/ enclosures:
Toledo Edison Company ATTN:
Mr. Lowell E. Roe Vice President, Facilities Development Edison Plaza 300 Madison Avenue Toledo, Ohio 43652 Donald H. Hauser. Esq.
The Cleveland Electric Illuminating Cencany P. O. Dox 5000 Cleveland, Ohio 44101 Gerald Charnoff, Esq.
Shaw, Pittman, Potts and Trowbridge 1800 i; Street, N. W.
Washington, D. C.
20036 Leslie Henry, Esq.
Fuller Seney, Henry and Hodge 300 Madison Avenue Toledo, Ohio 43604 DPf1;1.4!Ldl DPfhl.WR #1 o,,,e.,.
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ATTENDAtlCE LIST AUGUST 25, 1977 MEETING (AFTEPfiOON1 WITH THE TOLE 00 EDISON COMPANY DAVIS-BESSE, UtlIT 1 COCKET NO. 50-346 i
NRC - STAFF BECHTEL CORPORATION L. Engle P. Anas T. Ippolito V. Marathe F. Jablonski B. Novich R. McDermott L. Wise C. Mcdonald C. Miller F. Schroeder BABC0CK & WILCOX C0? patly K. Seyf ri t J. Stolz R. Donovan A. Szukiewicz E. Patterson T. Tambling G. Schieck R. Tedesco V. Thomas D. Vassallo SHAW PITTfMN, FOTTS ?. TROWBRIDGE P. Wagnar G. Charnoff TCLECO EDISON COMPAtlY R. Bins C. Domeck F. Millar L. Roe A. Topor orres e guese&ast &
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i MEETING
SUMMARY
Docket Fil NRC POR Local POR 4
TIC s.
NRR Reading LWR-#1 File E. Case R. Boyd R. DeYoung D. Vassallo J. Stolz K. Kniel
- 0. Parr S. Varga L. Crocker D. Crutchfield F. Williams R. Mattson H. Denton D. Muller Project Manager: L. Engle Attorney, ELD E. Hylton IE (3 )
ACRS (16)
L. Dreher NRC
Participants:
L. Engle T. Ippolito i
F. Jablonski i
R. McDermott C. Mcdonald C. Miller F. Schroeder i
K. Seyfrit J. Stolz A. Szukiewicz T. Tambling R. Tedesco V. Thomas i
D. Vassallo P. Wagner i
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ENCLOSURE 2 l
DAVIS-BESSE UNIT 1 STATION AND INSTRUMENT GROUNDING 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 minimum of every other building coltan. Horizontal conductors are installed as needed to tie equipment to the station ground grid (20' to 70' horizontally). The station ground grid i
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.
The instrumentation common siznal and analog signal cable shields for the following major systems are tied to the instrument ground bus:
1.
Reactor Protection System (RCS).
2.
Integrated Control System (ICS).
3.
Non-Nuclear Instrementation (NNI).
4.
Computer processing unit, local and remote multiplexers, typers, line printer, paper tape punch / reader, card reader, cathode ray tubes and operator keycoards.
5.
Miscellaneous electronic' control system.
The analog signal cable shields for the following major syste=s are tied to the instrument ground bus:
1.
Safety Features Acutation System (SFAS) 2.
Steam and Feedwater Rupture Ccatrol System (SFRCS) l 3.
Steam generator level l
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l The station and instrument grounding systems have been installed in accordance with the Davis-Besse Uni I design criteria. During the testing on these systems, the following has been found:
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RPS, ICS and NNI; the instrument ground buses in these systems,are intentionally tied together. There may be an inadvertent tie between this group of instrument ground buses and either the station ground system or the instrument ground bus in the computer or the miscells rous electronic control system.
"2.
There is an inadvertent tie between the following in-strument ground bus and either the station ground system or the instru=ent ground bus in another system:
Computer.
A.
B.
Miscellaneous electror.ic ccatrol system.
EQUIPMENT 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 i
B&W units in operation with these two buses tied together in the cabinets and then connected to the station ground grid. The RPS is designed to operate within specifications when installed with a single grounding system or with separate station and instrument grounds as specified at Davis-Besse. The Davis-3 esse grounding system meets the B&W balance-of plant (BOP) requirements f:: this
. installation. The S&W BOP requirements stated in the 205 BSAR call for a grounding system similar to that employed at Davis-Besse.
i The NNI and ICS were designed to have either single or separate in-strument and station ground systems. All B&W units in operation have had the two ground buses in these systems tied together. The NNI and ICS cabinets and interconnecting cables to remote 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 represeutatives to =odify the equipment to separate the ground buses in these systems.
The computer and miscellaneous electronic control system were designed and shipped with separate instrument and station ground systems.
The SFAS and SFRCS and steam generator level syste=s were designed and shipped with separate instrument and station ground systems.
Only analog signal cable shields are connected to the instrument ground bus. Noise tests conducted by their manufacturer (Ccasolidated Controls Corporation) indicated that these shields could be connected to eith,er the station or instrument ground system or they could be left ungrounded.
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MAXIMUM GROUND FAULT CURRENTS If a ground fault on any of the major electrical equipment, station power electrical buses or transformers should occur, this fault current would flow through the station ground system. The maximum ground fault current that could occur due to a fault on each elec-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 resistor.
13.8 KV Housepower buses *- 400 amps.
Limited by ground resistor.
4.
4.16 KV Housepower buses - 400 amps. Limited by ground resistor.
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480 Volt Housepower buses - 21,900 amps.
6.
250/125 Volt DC Housepower buses are ungrounded and no DC fault currents can flow in the station ground grid.
MAXIMUM GROUND CURRENT INDUCED VOLTAGES If there is an inadvertent tie between the instrument ground sys'em t
in the RPS and the station ground, loop fault current from an elec-trical fault could flow in the RPS instrument ground connection producing an induced voltage at the RPS system.
The largest ground currents that can flow thorough the station ground system in the Cable Spreading Room ceiling would be caused by an ground fault in the 430 V Motor Control Center (MCC) located directly above the Control / Cabinet Room at elevation 643'.
The maximum ground fault current that can occur at this MCC is 12,600 amps assuming zero ohms station ground syste= impedance. The source of shis ground fault current is a 1000 KVA transformer located on 21evation 603' at the northside of the auxiliary building.
This 1000 KVA transfor=er has a 6.75% impedance on 1 MVA base, 13.3 Y) transformer with a solidly grounded KVA to 480 V, (Delta :
neutral. The 13.5 KVA bus is assumed to have zero impedance up to the 480 volt transformer.
In calculating this induced voltage that could be i= pressed on the RPS instrument ground bus, the following conservative assumptions were made:
1.
Only 50% of the ground fault current would flow from the MCC to the 1000 KVA transformer on the ground cables installed in the conduits with the phase cables.
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The station ground grid at elevation 643' is assumed connected to the 1000 KVA transfooner by a single 500 MCM uninsulated copper cable which forms a 165' by 40' vertical boundary of the auxiliary building region involved. All the fault current which flows in the station ground is assumed to flow through the Cable Spreading Room ceiling in an East-West direction, and has been taken to be a single 500 MCM cable.
3.
No ground f ault current would be by-passed through building steel.
4.
The location of the inadvertent tie between the instrument and station ground systems was picked in an attempt to maximize the loop currents flowing in the RPS instru=ent ground buses.
5.
A zero voltage drop in the station ground system between the 1000 KVA transformer and the point where the 500 MCM insulated conductor from the instrument ground bus ia the Cable Spreading Room connects to the station ground system. This assumption will further maximize the fault current in the instrument ground loop in the RFS cabinets.
Using the above conservative assumptions, the fault voltage that could be i= pressed on the instrument ground buses would be 1.2 volts, ras, 60 Hz.
The ground fault current that caused this voltage would be interrupted within 3 cycles (50 ms.).
This 1.2 volt difference between NI/RPS cabinets would cause 60 Hz current flow thorough the system's instru=ent ground. Normally, this kind of " noise" could enter the system through inductive or capac:tive coupling between a ground lead carrying the noise and an analog signal lead. 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 translates to a 2500:1 power attenuation or a 50:1 voltage attenuation. With 34 db rejection, the worst ef fect 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 =odules prior to the bi-stable set point module, additional attenuation occurs reducing the error to less than 0.06%.
Considering the assumed direct coupiinz, the effect is extremely conservative.
It is 3&W's accessment that this is not a safety concern.
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MEETING
SUMMARY
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Docket File M.
NRC PDR Local POR TIC NRR Reading LWR-#1 File E. Case R. Boyd I
R. DeYoung D. Vassallo J. Stolz i
K. Kniel
- 0. Parr S. Varga L. Crocker I
D. Crutchfield F. Williams R. Mattson H. Denton D. Muller Project Manager:
Attorney, ELD E. Hylton l
L. Dreher i
NRC
Participants:
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