Thank you Stuart. We have not spoken for quite a while.
Yes, I used Rewrite, exp when Mathcad 15 took forever producing an s^30 series. I have a new, very fast, HP desktop machine with heat pipe board cooling and an added 40GB solid state hard drive. It was made for ultimate gaming but my long series take an awful time, probably because of the cosh/sinh arguements. There were sign changes and I was afraid of getting into some non- converging region. I rationalised the awfully long quotient and used Solve to pull-out one root at a time. it was tedious but steered clear of convergence problems, real or imagined! Hopefully I can ignore RHP poles because they only exist before the fault happens. There is no feedback, just standing waves.
Richard ( thankyou) has assured me that my fears were groundless and I do not need 17 digit accuracy.
I have invented an inexpensive way to control and prevent large surge currents and high voltages in a shorted power feed. Carelessness or a cannon shell may trip a circuit breaker, but there will be no arcing damage or fuel ignition.
This calculation was eased by a greatly simplified solution of the transmission line equations, for this application, which I produced when I retired from EMC work in the JSF Program Office at Pax.