Estimating Failure Rate: Chi-Square v Poisson

I understand Poisson distribution is for discrete events and can be used to estimate failure rate to a given confidence level from elapsed time and number of failures (assuming constant failure rate).

However, in safety system engineering I routinely see the Chi-squared distribution used. Is this just because it is better known? What are the pros & cons here? How are degrees of freedom identified for chi-sq?

My thanks in anticipation.