You've presented a scenario that requires careful consideration. Let's break down the benefits of using fault indicators in manholes along an un-spliced MV feeder cable, even when it's a direct run from substation to unit substation

Potential Benefits of Fault Indicators in Manholes (even with un-spliced cable):
Fault Location Speed (Generally Reduced, but still applicable)

Even without splices, cable faults can occur along the length of the cable within a manhole. A rock could compromise insulation, water ingress could occur, or manufacturing defects could become apparent over time. Fault indicators, even if they only narrow the fault down to a section between manholes (or from substation to first manhole, or last manhole to the unit sub), this is still faster than TDR (Time Domain Reflectometry) for locating the fault on the entire cable length. TDR is still often required for exact location, but indicators shrink the distance the TDR has to analyze. By halving or quartering the length of cable that needs to be investigated, you can significantly reduce the excavation and repair time.

Reduced Outage Time

Faster fault location translates to faster restoration of power. This is crucial for critical loads.

Improved System Reliability Data

Fault indicators provide data on the frequency and location of faults. This information can be valuable for: Identifying weak points in the cable system. Evaluating the performance of cable insulation. Optimizing maintenance schedules.

Safety

Although less direct than with spliced cables, knowing which section of cable has the fault before you start digging or working in a manhole improves safety. It reduces the risk of accidentally cutting into a live, faulted cable.

Phased Installation

Even if you don't install indicators in every manhole initially, having the capacity to add them later as budgets permit is a good strategy.

Considerations that Might Reduce the Benefits



Cost

Fault indicators add to the initial cost of the installation. The cost-benefit analysis should consider the potential savings from reduced outage time and repair costs weighed against the upfront investment.

Maintenance

Fault indicators, especially electronic types, require periodic maintenance (e.g., battery replacement, sensor checks). This adds to the ongoing operational costs. Ensure that maintenance schedules are practical and well-defined.

Indicator Type

There are different types of fault indicators (e.g., current-only, voltage-and-current). Choose the type that is best suited for the application and system protection scheme. Current-only indicators are probably the most appropriate here, as they're triggered by overcurrent and do not require voltage input.

Environmental Conditions

Manholes are often harsh environments with moisture, temperature extremes, and potential for corrosion. Choose fault indicators that are designed to withstand these conditions.

Coordination with Protection Scheme

It's crucial that the fault indicators are properly coordinated with the overall protection scheme of the substation switchgear breaker and unit substation switch. False indications or missed faults can disrupt the system and create confusion. In particular, consider the time-current characteristics (TCC) curves of the breaker and fuses.

Here's a framework for making a decision


Assess the criticality of the load

How much does an outage cost in terms of lost production, revenue, or other impacts? A highly critical load justifies a greater investment in fault location equipment.2.

Analyze the historical fault data

Are cable faults a common occurrence in this area? If so, fault indicators are likely to be beneficial. If cable faults are extremely rare, the benefits are reduced.3.

Evaluate the cost of alternative fault location methods

How long would it take to locate a fault using TDR alone? What are the labor costs involved?4.

Consider future expansion

Will the cable system be expanded in the future? If so, fault indicators can be a valuable asset for managing a larger network.5.

Weigh the costs and benefits

Prepare a cost-benefit analysis that compares the cost of installing and maintaining fault indicators with the potential savings from reduced outage time and repair costs.

Recommendation

In your scenario, with an un-spliced cable, the benefits are less dramatic than with spliced cables. However, I would lean towards

installing fault indicators, especially if the load is critical or if cable faults have been a problem in the past. Start with indicators in the first and last manholes. This will immediately tell you if the fault is in the "first section" (substation to first manhole), the "last section" (last manhole to unit sub), or somewhere in the middle. This is a lower cost and still provides a large improvement over TDR alone. If budget is available, include indicators in all manholes.

Alternative Strategy

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Prepare the manholes for future installation

Install mounting provisions and conduit runs for fault indicator cables during the initial cable installation. This will make it easier and less expensive to add fault indicators later if the need arises.

Important Note: Thoroughly document the location and type of each fault indicator for future reference.By carefully considering these factors, you can make an informed decision about whether or not to install fault indicators in your MV feeder cable system. Flag for review