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Technical Bulletin
A Comparison of Three Different Methods of Making Primary Connections
Summary
Three different methods of connection are compared, two types of separable connections, standard FAA, and modified FAA (flying leads), and permanent connections. The prime consideration is expected reliability, but material cost, labor cost, and operational issues are also considered. We conclude that:
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The standard FAA method is the most advantageous overall. |
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Permanent connections may have better reliability than the standard FAA method, but are more expensive and much less convenient for troubleshooting. |
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The modified FAA method is inherently the least reliable as well as being the most expensive. |
Expected Reliability
This is not intended to be a definitive comparison based on field experience (does anyone keep records detailed enough to do such a comparison?). Nor is it quantitative, using concepts such as MTBF. Rather, it takes a qualitative approach, using the following rules of thumb:
Conductor
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Reliability decreases as the number of connections increases |
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Crimping done in a factory is likely to be more reliable than that done in the field. |
Insulation
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Reliability decreases as the number of connections increases |
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Factory interfaces (whether molded or premolded) will be more reliable than those formed in the field (e.g. cast epoxy, heat shrink). |
Expected Reliability Comparison
Method
(click for
circuit diagram) |
Conductor |
Insulation |
Total |
| Permanent Splices |
Separable Connectors |
Field |
Factory |
| .Field |
Factory |
Premolded |
Molded |
| Standard FAA |
2 |
2 |
2 |
. |
2 |
4 |
12 |
| Modified FAA |
4 |
4 |
2 |
4 |
. |
4 |
18 |
| Permanent |
4 |
. |
. |
4 |
. |
. |
8 |
Discussion of Reliability
A cursory look might take the total column as a rough index of overall reliability. On this count the permanent splice method looks to be the most reliable, and the standard FAA method less so. A more careful look shows that the number of field connections and interfaces for the permanent method are considerably higher than those for the standard FAA, so it is an open question whether this method will be consistently more reliable being more highly dependent on installer skill. Experience also shows that permanent connections are much more prone to failure from water that gets between the cable insulation and jacket, and wicks into the transformer. Connectors provide a natural water block to almost eliminate this possibility. The modified FAA method clearly has the lowest expected reliability.
Permanent or Separable Connections?
This is generally considered as trading off higher reliability (permanent splices) for more difficulty in trouble shooting. The expected reliability analysis above casts some doubt on whether the reliability of permanent connections can really be expected to be significantly better than that of the standard FAA method, whereas difficulty for trouble shooting is assured.
Why Use the Modified FAA Method?
We are familiar with two reasons given for using this method instead of the standard FAA.
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That the factory molded connectors on the flying leads will be more reliable than premolded connections (on the kits). We would not disagree in principal, but a look at the analysis above shows that this comes at a heavy cost, i.e. by introducing 4 extra connections in the conductor, and 2 extra interfaces between insulation. |
| 2. |
That with very stiff primary cable, even the flexible leads on the transformers are not enough to ensure a suitable installation, and therefore additional flexibility from the flying leads is desirable. We suggest that for unscreened cables, even the notoriously stiff XLPE, this is rarely necessary. For screened cables lack of flexibility in the primary cables can be more of a problem. In either case, if more flexibility is desired, we suggest that other solutions such as longer primary leads on the transformers, or the use of more flexible primary cable insulation such as EPR should be considered, rather than compromising circuit reliability. |
Overall Comparisons
Connection
Method
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Reliability
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Material Cost
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Field Labor Cost
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Ease of troubleshooting
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Ease of Use with Stiff Primary Cables
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Susceptability to water ingress |
| Standard FAA |
Medium? |
Medium |
Low |
High |
Low (Note 1) |
Low |
| Modified FAA |
Low |
High |
High |
High |
High |
Medium |
| Permanent |
High |
Low |
Medium |
Low |
Low |
High |
| Note 1: |
Other solutions such as longer primary leads on the transformers, or more flexible primary cables (e.g. the use of EPR insulation) are available. |
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