From my experience, it can sometimes take time and effort to get the required information and so it is recommended that the very first thing that is put in place is an enquiry letter. The following is a sample enquiry letter.
Sample Enquiry Letter
To - Electricity Company & Address
Electricity Safety Risk Assessment – Request for Supply Characteristics Information
Site Address and intake substation reference number (or customer reference number)
We are currently carrying out an arc flash risk assessment and request your cooperation in order to complete the work. This work is essential to develop strategies to protect our personnel and your own whilst on site against the dangerous effects of arcing faults on our equipment. The work will involve a protection grading and an arc flash hazard study. In order to perform the calculations, we will require fault level and protection co-ordination information at the point of connection to our site. The information requested is as follows.
- Maximum and minimum symmetrical prospective short circuit fault current at the supply terminals.
- X/R values at maximum and minimum fault current.
- Maximum and minimum earth fault current levels.
- The type, rating and settings of the protective device or devices nearest to the supply terminals.
Please be aware that the study will require the minimum fault currents as well as the maximum as lower fault currents can create the highest level of hazard in some circumstances. I will be pleased to discuss our requirements in greater detail and my contact details are as follows.
Name, address email and telephone number.
Thank you in anticipation of your cooperation and I look forward to hearing from you in due course.
Yours faithfully etc...
You will notice that I have deliberately kept the information as simple as I can and by not overcomplicating the message with additional non-essential information. Once contact has been made with a technical engineer who can help, more nuanced information can be sought if necessary. Experience has shown me that once contact with the right person has been made, engineers are usually very helpful particularly when they are speaking to fellow engineers.
The letter should go out on the company letterhead for whom the supply contract exists and not on letterheads from third party contractors or consultants. An alternative is to get an accompanying letter of authorisation signed from the company from whom the supply contract exists if you are acting on their behalf.
You will also notice that I have not included the units in the enquiry. It is likely that different supply companies will present their data in different ways. For instance, the information may come back as an R+jX impedance or MVA or kA plus X/R ratio. The main thing is not to place barriers in the way of the response.
Once the source prospective short circuit current is known then calculations can be made to determine the short circuit current at the equipment. One of the calculators which is appended to this guide will facilitate this. Given the source high voltage short circuit current and X/R ratio on the transformer primary, PSCC can easily be calculated anywhere in the low voltage network.