Understanding Voltage Threshold Ratings

All solid-state DC decouplers and Over-Voltage Protectors have specific voltage threshold ratings. For successful application, it is important to understand these ratings and how they can affect your CP system. This article will use specific examples to explain how to select the proper voltage threshold for your application.

Decouplers and Over-Voltage Protectors are designed to block the flow of low voltage DC current yet provide safety grounding protection in the event of AC faults or lightning. The voltage threshold (also known as the blocking voltage) of the device determines the voltage across the terminals at which the device transitions from blocking mode (DC isolation) to conduction mode (safety grounding). When the voltage across the device terminals exceeds the voltage threshold, in either polarity, the device will conduct both AC and DC current. Once the voltage drops below the voltage threshold, the unit will return to blocking mode.

Common threshold voltage ratings are written as -2/+2 or -3/+1. This is the voltage range (i.e., between -2V and +2V) within which the device blocks DC current. The table below gives examples of how threshold voltage ratings are specified in the model number of different Dairyland devices.

Though -2/2 and -3/1 are typical ratings, some devices are also available with higher voltage thresholds for use in applications with higher CP levels.

How to determine the proper voltage threshold rating

To determine an appropriate threshold rating, measure the DC voltage across the points where the device will be connected during normal operating conditions. This voltage can also be estimated by calculating the difference between the pipe-to-soil potentials from each side of where the device will be connected. In either case, record the voltage as the negative terminal voltage with reference to the positive terminal voltage. This voltage should be within the range of the device voltage threshold rating.

Although decouplers are designed to provide low impedance for AC current, there will be some small AC voltage drop across the device if any AC current is present, and it is important to account for this added voltage drop when selecting a voltage threshold rating. For example, for SSDs and standard PCRs, the device impedance is approximately 0.010 Ohm at 60 Hz and 0.012 Ohm at 50 Hz. Induced current of 10 Arms flowing through an SSD at 60 Hz will create 0.10Vrms (0.14 Vpeak) of AC voltage drop across the SSD. Be sure to add the peak value of this AC voltage drop to the DC voltage difference expected across the device. Note that at the maximum rated steady state current for SSDs and standard PCRs of 45Arms (64Apeak), this voltage drop due to AC could be as high as 0.64V at 60 Hz (0.76V at 50 Hz).

Examples

Scenario 1:

Both sides of an isolation joint are protected by CP. The P/S (pipe-to-soil voltage) ON reading for one side of the joint is -1.35V. The opposite side of the joint has an ON reading of -1.05V, resulting in a voltage difference of -0.30V. There is no induced AC measured on the pipe.

Since the nominal DC voltage of -0.30V across the joint is well centered within both -3/+1 and -2/+2 ranges, either rating would be appropriate. This is illustrated below in the graph of voltage vs. current for a decoupler with -2/+2V rating. Note that the DC leakage current through the decoupler is less than 3 mA within the voltage threshold range.

Typically, however, a symmetrical device is used for the situation where CP is present on both sides of an isolation joint since the difference in potentials across the joint is typically close to zero.

 

 

 

Total Voltage expected across device terminals = V_DC + V_AC

V_DC = V₁ – V₂
V₁ = -1.35V
V₂ = -1.05V
-1.35 – (-1.05) = -0.30V
V_AC = 0

V_DC + V_AC = -0.30 + 0 = -0.30V

This is well within the threshold rating of a -2/+2 decoupler.

Scenario 2:

The cathodically protected side of isolation joint P/S ON reading is -2.45V. The opposite side of the joint is grounded to copper with a corrosion potential of -0.34V. There is no induced AC measured on the pipe.

The DC voltage difference is -2.11V which exceeds the negative threshold of a -2/+2V rated device. As shown in the graph below, using a device with a -2/+2 voltage threshold in this application would cause the device to be continuously in conduction and so short out the structure to ground.

Total Voltage expected across device terminals = V_DC + V_AC

V_DC = V₁ – V₂
V₁ = -2.45V
V₂ = -0.34V
-2.45 – (-0.34) = -2.11V
V_AC = 0

V_DC + V_AC = -2.11 + 0 = -2.11V

This exceeds the negative threshold of a -2/+2 device and would result in a continuous short to ground.

Recommendation: Select a device with a threshold rating of at least -3/+1V. See the graph below for the new, acceptable result if a -3V/+1V device were used instead of a -2/+2V unit.

 

Scenario 3:

Decouplers to be installed to isolate a pipeline from an AC mitigation grounding system. The pipeline P/S ON reading is -2.25V. The copper mitigation wire reading is -0.35V. The maximum steady state induced AC current flowing from the pipe to the mitigation wire at this location is 10Arms at 60 Hz.

The DC voltage difference is -1.90V, which is close to exceeding the negative threshold of a decoupler with a -2/+2V threshold. The additional voltage drop due to the induced AC current is 0.10Vrms (0.14Vpeak). Combined, the DC and AC voltages across the decoupler will cycle between -2.04V and -1.76V at 60 Hz as shown in the graph below. In this case, the decoupler would cycle in and out of conduction at 60 Hz.

Total Voltage expected across device terminals = V_DC + V_AC

V_DC = V₁ – V₂
V₁ = -2.25V
V₂ = -0.35V
V_DC = -2.25 – (-0.35) = -1.90V
V_AC = Current * Decoupler Impedance

Current = 10Arms = 14.14Apeak

Impedance = 0.01 Ohm at 60Hz

V_ACPeak = 14.14Apeak * 0.01 Ohm

V_ACPeak = 0.14Vpeak

Vtotal = V_DC + V_AC = -1.90 +/- 0.14 Vpeak

Vtotal = -2.04V to -1.76V

This total voltage exceeds the negative voltage threshold of a -2/+2 rated device and would cause the device to cycle in and out of conduction at 60 Hz.

Recommendation:  Select a decoupler with a threshold rating of at least -3/+1V.

Device Operation with a Given Threshold Rating

The device should always be installed with the negative terminal connected to the more electro-negative structure – typically the cathodically protected structure – and the positive terminal to the more electro-positive structure, often a grounding system without cathodic protection. You can learn more about proper configuration of the terminals in the article Polarity Matters.

With this arrangement, the structure connected to the negative terminal will be limited to a voltage equal to the negative threshold rating relative to the positive terminal/structure. Likewise, the same negative structure cannot rise in voltage above the positive threshold, relative to the positive structure. This keeps one structure close in voltage to the other structure, regardless of which was affected by an over-voltage condition.

Rule of Thumb for Proper Threshold Selection

• In most situations where both terminal connection points are cathodically protected, a threshold rating of -2/+2 will be an acceptable choice. If one side has a CP reading that is higher than normal, then a -3/+1 or higher value should be selected.
• For isolation joints with only one side receiving cathodic protection, a threshold rating of -3/+1 is usually appropriate.
• For AC mitigation projects with copper grounding systems, a threshold rating of -3/+1 is usually appropriate. For AC mitigation systems with zinc grounding systems, a -2/+2 threshold rating is usually appropriate.
• In the case of isolating electrical equipment on a CP system from a copper grounding system, a threshold rating of -3/+1 is typical, since copper is notably shifted on the galvanic series of metals compared to most protected structure potentials.
• When decoupling an entire facility with CP from a power utility grounding grid, the expected potential difference between two grounding systems is often small, and within the rating of -2/+2, however a -3/+1 threshold is also a safe choice.

PCRX Voltage Threshold Selection

The voltage threshold ratings for the PCRX differ slightly from other devices. See the PCRX Technical Literature for a detailed description of how to select the threshold voltage for a PCRX.

If selecting the proper threshold rating is in question, please contact Dairyland’s technical support.