ATLAS Policy on Grounding and Power Distribution

Introduction
As is well known from previous experiments, careful attention must be paid
to the grounding and power distribution of each of the detector systems ifthey are to operate successfully at the low signal levels required. While
this has been an important issue in previous experiments, it is an especially
important issue for ATLAS given the large expense and time required to redo
any system, and given the very large power in most systems. In this note, weoutline the present ATLAS policy on grounding and power distribution. The
primary content of this note is a set of proposed guidelines that have been
arrived at via reasonably extensive consultation with each of thesubsystems. The intent is that these guidelines must be followed unlessspecific approval for a deviation is granted by the Executive Board. Some
discussion is also presented as to how the guidelines can be accomplished.
It is expected that recommended implementations will become increasingly
detailed after further discussion and thought and as experience in test
beams and system tests provides additional information.

A Summary of ATLAS Policy
The primary guidelines are the following. First a summary list is presented
and then comments are presented on each item.
•All Detector Systems will be electrically Isolated
No Connection to Ground other than "Safety Network"
No electrical connection between different detector systems•

Low Voltage Power Supplies Floating
Power return either fully isolated from Gnd or connected by device which
has high impedance for normal operation but low impedance during failure(e.g. saturable inductors or diodes)•High Voltage Power Supplies Floating
HV return likely isolated from detector by resistor•

Data, Clock, and Trigger Transmission

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Optical or Shielded Twisted Pair•

Detector in Faraday Cage•

Monitor and Control Signals
Optical, Shielded Twisted Pair, or similar isolation

Comments on Individual Guidelines
All Detector Systems Electrically Isolated - The electrical isolation really
has two different aspects: isolation from "Ground" and isolation of one
detector system from each of the others. The first aspect is concerned
primarily with the desire to eliminate, or at least minimize, ground loops.
Each system will have to be connected to earth at some location for safety
reasons, but the location and manner of this connection must be chosencarefully. In the last section of this document, the location and nature of
the proposed safety connections to earth are summarized for each of the
major subsystems. By designing the system so that it can in principle be"completely" isolated from earth (safety considerations aside), one has
maximum control over how the connection to earth is finally made, and thebest assurance that unintended connections are avoided.
Inadvertent or unintended connections between detector systems could wellfoil the intended noise immunity of a particular system and would certainly
make debugging noise problems much more difficult. In the final installation,
it could prove advantageous to connect some detector systems together, but any
such connections should be carefully designed and implemented only
optionally and with the agreement of all subsystems.
While parasitic AC "connections", either to ground or between detector
systems, can never be completely avoided, an effort should be made to identify
those that are likely to be problematic and to minimize them.
Low Voltage Power Supplies Floating - The focus here is on treatment of thepower supply return, whether it is connected to earth and if so in whatfashion. It is our present understanding of CERN safety policy that if thevoltage is less than 50 volts it is acceptable to have the power returns
completely isolated from earth at the power supply. For larger voltages, a"safety" connection of the power supply return may be required. In the eventthat a "safety ground" is required, for those sybsystems that do not intend to
make their "primary" earth connection at the power supplies, it will berequested that this connection be made via circuit elements which provide a"high" impedance to earth during normal operating conditions but a very low
impedance in the event of failure. Examples of such elements are "saturable
isolation inductors" and diodes; a schematic of the power supply
configurations, for voltages less than 50 volts and for larger voltages, ispresented in Figure 1.
High Voltage Power supplies Floating - Again the focus is on treatment of thepower supply return. Where truly high voltages are utilized (e.g. > 1000volts, and distribution is via coaxial cables, it is likely that the shieldPage 2

of the cable must be connected to earth. In this case it is customary to breakpotential ground loops by connecting the HV shield to the detector via aresistor, ypically of order 1 kilohm. (In some cases a resistor may also beinserted in the HV supply conductor, to provide a more balanced system). In
the initial plans, most subsystems in ATLAS plan on this traditional
approach. It may also be considered whether safety connections that provide
high impedance during normal operation, such as those discussed for low
voltage power supplies, will be allowed. An alternative approach could also
be to distribute the HV power and HV power return via two conductor cables
with an outer shield. In this case, grounding the outer shield should be
sufficient for safety purposes (this could also reduce noise conducted intothe detector). .
Data Transmission, Clock and Control Distribution - Transmission of the dataand clock and control is expected to be either via optical fiber or shielded
twisted pair using low level differential signals. If the latter is utilized,
it is expected that the shield of the cable will be connected directly to thelocal "detector ground" at the detector and via a capacitor to the local"ground" at the other end. This provides for maximum shielding and noise
immunity at high frequencies while at least breaking any ground loops createdat low frequencies. An example (certainly not unique) of a system which uses
shielded twisted pair in this fashion and obtains very good noise immunity isNA48; a diagram of their signal and ground connections is presented inFigures 2 and 3.
Detector Located in Faraday Cage - The maximum protection against EMF noiseis produced by locating the detector(s) in a carefully designed Faraday cage.
Currently this is planned for the Liquid Argon, Tile Calorimeter, TRT andMuon Systems. The SCT and Pixel systems are less susceptible to pickup thansome of the others, but some form of overall shielding may be necessary.

Plans for Implementation in each Subsystem
Each subsystem is required to provide a description how it intends toachieve the guidelines stated above. Key features of this note include:
Summary List of how electrical isolation is maintained for ALL
mechanical and electrical interfaces to the subsystem
Schematic diagram(s) indicating where, and in what manner, allconnections to earth or safety grounds are made.
Summary of primary outstanding issues, if any.
Identification of an individual to serve as primary contact for
information and meetings concerning grounding and power distribution.
Initial plans have been received for almost all of the subsystems, and also
for the magnet systems. Most of these are available on the WEB via theFront End Electronics link from the ATLAS page. An example summary of thetechniques used to achieve isolation is shown in Fig. 4 for the Liquid Argon
System. Additional details of the plans for implementation will be provided
during Fall 1998

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1. LV Supplies <50V
GFI
<50V~220VDC<50V secondary isallowed to float

Neutralsafety ground

2. HV Supplies > 50V~kV Supplies, low current

Cryostat~220 V
1-10 kΩ
Safety ground3. Supplementary safety grounding:high impedance at low voltage

Saturable inductor

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Planned Connections to Earth - Safety Grounds
For reasons of safety all conducting objects of any size in a subsystem must
be connected to earth. We note here that connections to earth will inpractice be made via a small number of cables which travel from the cavern
or USA15 or other halls to the surface building. Because of the poor
conductivity of the rock surrounding the cavern, there is no "local" earth
connection. As noted above, to eliminate ground loops there should be a"single point connection" for each piece of a subsystem that is electricallyisolated from other subsystems and from other pieces of the same subsystem.
(A "single point connection" could consist of a number of connections orconductors as long as the routing of the conductors and the location of theconductors does not allow significant loops.) Considerable discussion has
been held amongst the subsystems to determine where this connection would
be best made. The TRT, Liquid Argon Calorimeter, Tile Calorimeter, and Muon
systems currently endorse the following plan••The safety connection to earth will be made in USA15
The connection should be made via a well-defined number of connections orcables, such that the quality of the connection and the current flowing canbe easily monitored• There will be NO connection of the detectors to earth, including large
metallic objects such as the cryostat, in the cavern.
• A safety procedure should be established such that the safety connection in
USA15 cannot be removed unless an alternate safety connection has beenmade.
We note that this approach is nearly identical to that of the NA48 experiment.
Diagrams specifying the location and nature of connections to ground for
the NA48 experiment have already been presented in Figures 2 and 3. In thiscase the primary connection to safety ground is made at the CPD crates which
receive signals sent from the calorimeter. There are no additional
connections to ground either at the cryostat or at the power supply returnleads at the preamp supplies.
Common aspects of the TRT, Liquid Argon Calorimeter, Tile calorimeter, andMuon systems that argue for the above approach are (1) it is expected that thepower supplies will be located in the cavern in a number of different
locations and (2) there will be a significant number of signals sent overtwisted pair to USA15. Thus making safety grounds at the power supplies
would create ground loops (unless high impedance safety connections wereutilized). There is also a strong argument to reduce any potential
difference between the "local grounds" of the receiving electronics and the"local detector ground" at the electronics on the detector. Making the earth
connection for the subsystems, at USA 15, accomplishes this since little orno DC current should be flowing in the ground connection between thedetectors and USA 15.
For the SCT and Pixel systems a different approach is anticipated. In thecase of these systems (1) it is planned to locate all power supplies either
in USA15 or US15, (2) all transmission to USA15 is optical, (3) the detector
systems are highly modular encompassing 4080 (2500) isolated modules. Eachof these modules has an independent power supply. Thus it is planned thatthe safety connection will be made at the power supply for each module.

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Page 650

50

REEMITTERBOX∼ 10m

Figure: 2300µH

39p

100

39p510

DIGITAL GND

ANALOG GNDCPDFAST BUS CASERECPage 7SAFETY GNDPRIMARYFigure: 3

SECONDARYPREAMPPOWER
SUPPLYGROUNDED IN C.R. (C.P.D. RACK)LAr Cal. Vital lines (i.e., Potential ground loops)
1. Lar cryo lines
2. HV supplies
3. Data (opt.fibers)
4. Cooling circuit (water)

∼ 20m

FEE

5. LV supplies ≈ 30µΩ

∼ 70m
6. Level 1 sums4
∼ 5 ×10 channels
7. clock

Accordionwarm

8. slow controls(parameter input)cold

9. sensors
10. solenoid cryo & supply
1., 4., 11. insulators
11. mechanical supports

2., 5., floating supplies
6., transformers (balun, signal)

12. feedthrough heater (dc)

7., 8. opto-couplers (or transformers)
9. insulate sensors; different techniques at various receivers
10. solenoid line to be insulated, power supply floating
BUT .... a major effort required
12. heater insulated

Figure: 4Page 8

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