2. Continuity Test of the VTs installed on the superconducting magnets

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Tags/Project: Test-Procedures-wiki

Metadata Field	Document Information
Status	Draft
Created	2026-09-24
Last Updated	2026-10-01
Author	Davide Cuneo
Rev. #	0.4
Reviewers	TBD

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Introduction and Context

This procedure describes the verification of the Voltage Taps (VTs) connections on the magnet coils by means of a continuity test. The main goal is to check the effectiveness of the VT routing and to ensure that there are no open circuits or labelling mistakes before proceeding with the testing campaign. A wrong or missing VT would compromise both the quench detection and the protection of the magnet, so this check must never be skipped.

The test must be performed in three configurations:

1. Magnet standalone, as soon as it is received;
2. Magnet mounted on the insert, to verify the wiring from the coil to the insert connectors;
3. Insert connected to the PotAim crate, to verify that the signals correctly reach the acquisition and protection system.

The values measured in the first configuration are the baseline against which the following ones are compared.

As an example, the MCBRD9 corrector magnet tested at CERN is examined. Clearly, the operating steps will have to be adapted to the magnet under test (number of apertures, number of VTs, nomenclature).

Few words about the magnet and test

The MCBRD9 is a two-aperture orbit corrector magnet built in a Canted Cosine Theta (CCT) configuration with NbTi conductor. Each aperture is an independent circuit, electrically insulated from the other one.

The VTs are named according to the following convention:

• the first digit (1 or 2) identifies the aperture (AP1 or AP2);
• the second digit is the VT index along the coil.

For example, V13 is the third VT of AP1, while V21 is the first VT of AP2. Each aperture has at least two pairs of VTs on the current leads, plus several VTs dedicated to coil protection (six per aperture in this example).

The test consists of two parts:

1. A preliminary resistance check with a multimeter, without any current in the magnet;
2. A continuity test with voltage drop mapping, in which a small constant current is injected in one aperture at a time and the voltage across each coil segment is measured.

Since a constant current flows through the whole aperture, the voltage measured across each segment is proportional to its resistance ($V=R\cdot I$). Moving along the coil, the voltage must therefore change progressively, which confirms both the continuity of the coil and the correct order of the VTs.

Required Material

To carry out the test the following instrumentation and items are needed:

1. Multimeter (analog in the reference test) for the preliminary resistance and open-circuit check;
2. Digital multimeter for the voltage drop measurements;
3. External power supply able to provide a stable and precise low current, with an adjustable voltage limit, and able to withstand the possible inductive transients of the magnet;
4. Burndy connection box (available in FREIA), specifically made for the Burndy connectors of the VTs, which routes each VT to a standard banana jack;
5. Cables with banana connectors for the power supply and for the measurements;
6. Dedicated test card for the PotAim crate (only for the third configuration, see Figure 2);
7. The VT wiring scheme of the magnet and the mapping of the VTs to the PotAim channels.

Test preparation

1. Make sure that the magnet is not connected to any other power source and that no current is flowing in it.
2. Identify all the VT wires coming from the coil and check that each one is properly labelled according to the nomenclature above. The magnet usually arrives with labelled VT wires, but the labels must still be verified.
3. Take note of the ambient temperature and of the magnet temperature, since the resistance of the coil, and therefore the measured voltages, depend on it.
4. Prepare the table for recording the results (see the TestDoc file referring to the magnet to be filled).

Phase 1: Preliminary Resistance Check

Main objective: verify the physical routing and labelling of the VTs coming from the coil before powering the system.

With the multimeter, measure the resistance between the VTs of each aperture. Between two VTs of the same aperture a finite resistance must be measured.

About wire verification

If the multimeter shows an open circuit, the two VTs either belong to different apertures or are not properly connected. Always perform this sanity check first, to avoid troubleshooting mistakes later under current.

Phase 2: Wire Routing to Power Supply and Box

Main objective: set up the probing current loop and prepare the voltage measurement points.

1. Power supply connection. Connect the two polarities of the power supply directly to the VTs on the current leads of the aperture under test.

Power supply voltage limit

Even with a small current, this type of test can lead to high voltage values, for example in case of an open circuit or of a sudden change of current in an inductive load. Make sure that the power supply is adequate and that its voltage limit is set properly, to prevent safety hazards or damage to the magnet and the equipment.

2. Signal routing via the box. Plug the Burndy connector carrying the VTs of the magnet into the dedicated test box. Thanks to the internal routing of the box, it is straightforward to locate where each VT arrives. The voltage between individual segments can then be probed with standard banana connectors on the box.

Phase 3: Continuity Test and Voltage Drop Measurement

Main objective: inject current and map the voltage drop across all coil segments.

1. Current injection. Set the power supply to a constant current of 50 mA and switch on its output.
2. Voltage drop mapping. Measure and record the voltage across each segment, in mV. Moving along the coil segments, the voltage must drop progressively, confirming the continuity of the coil and the correct order of the VTs.
3. Consistency check. The sum of the segment voltages should match the total voltage measured between the first and the last VT of the aperture.

An example of the results obtained on the MCBRD9 magnet standalone is shown in Figure 1.

The acceptance criteria are the following:

• no open circuit on any segment;
• the voltage changes progressively along the coil, in the order given by the VT indices;
• in the second and third configurations, the measured voltages match the baseline values recorded with the magnet standalone.

Phase 4: Safe Ramp-Down and Shutdown

Main objective: safely de-energise the magnet without generating dangerous voltage transients.

Once the measurements are completed, bring the current to zero and switch off the output directly from the power supply interface or device control. Only then disconnect the cables.

Disconnecting wires

Never stop the test by pulling out the wires while current is flowing. The sudden current drop (high $dI/dt$) in the inductance of the coil induces a high voltage spike that can damage the magnet or the instrumentation.

Repeating the Test

Once the first aperture is complete, the same procedure (Phases 2 to 4) is repeated for the other aperture, connecting the power supply to the lead VTs of AP2 and measuring the EE2x segments. All other parameters remain unchanged.

The whole test is then repeated once the magnet is mounted on the insert, and the results are compared with the standalone baseline.

PotAim Crate Wiring Verification

Main objective: check the connection of the wires coming from the insert to the PotAim crate.

This check is performed with the adapter card, shown in Figure 2.

Test card functionality

The test card intercepts the signals coming from the insert to the PotAim crate and provides direct access to the individual channels at the specific slot where it is inserted.

1. Insert the test card into one slot of the PotAim crate. The wiring of the signals to the PotAim channels must be known in advance.
2. Inject again a constant current of 50 mA into the magnet leads using the external low-current power supply.
3. Measure the voltage drop across the exposed channels of the test card using the digital multimeter. The measured segment voltages must match the baseline values recorded during the initial continuity test.
4. Bring the current to zero from the power supply before moving the test card
5. Repeat the test for each slot used in the PotAim crate.

Direct or differential mode

Pay attention to the input mode (direct or differential) set on each PotAim channel, since it affects the measurement being performed. The mode is set by the input jumpers of the card.

Recording the Results

For each aperture, record the measured voltages in a tablein the Excel TestDoc, so that the three configurations can be compared directly. Also record the date, the operator, the ambient and magnet temperature, and the voltage limit set on the power supply.

After the Test

1. Make sure the power supply output is off and no current is flowing.
2. Disconnect the power supply from the lead VTs.
3. Unplug the Burndy connector from the test box, or remove the test card from the PotAim crate and put back the PotAim card in its slot.
4. Save the results table and compare it with the previous configurations.

Operational Checklist

To be filled in during test execution, in order.

Preparation

• Verified that no current is flowing in the magnet and that it is not connected to other power sources
• Identified and checked the labels of all VT wires
• Taken note of ambient temperature and magnet temperature

Phase 1: Preliminary resistance check

• Measured the resistance between the VTs of AP1 (no open circuits)
• Measured the resistance between the VTs of AP2 (no open circuits)
• Verified that there is no continuity between AP1 and AP2 VTs

Phase 2: Wire routing

• Connected the power supply to the lead VTs of the aperture under test
• Set the voltage limit of the power supply
• Plugged the Burndy connector into the test box

Phase 3: Continuity test

• Injected 50 mA constant current
• Measured and recorded the voltage of each segment
• Verified the progressive voltage drop and the sum of the segments against the total

Phase 4: Shutdown

• Brought the current to zero and switched off the output from the power supply
• Disconnected the cables only after the current was zero

Repetitions

• Repeated Phases 2 to 4 for the other aperture
• Repeated the whole test with the magnet mounted on the insert and compared with the baseline

PotAim crate verification

• Checked the input mode (direct or differential) of the channels
• Inserted the test card in the first slot used
• Injected 50 mA and measured the voltages on the test card channels
• Compared the values with the baseline
• Brought the current to zero before moving the test card
• Repeated for each slot used in the PotAim crate

After the test

• Power supply off and disconnected
• Burndy connector unplugged / test card removed and PotAim card back in its slot
• Results table saved

Feedback

Found an error or something outdated in this note? Please contact Davide Cuneo (davide.cuneo@physics.uu.se).