As the previous responses indicate you need to exercise care when carrying out ground tests on ATC Transponders, particularly Mode C as there has been a number of false warnings with TCAS equipped aircraft in the vicinity when testing is underway.
CAA has published guidance on ground testing of ATC Transponders in CAAIP (CAP562). It is well hidden but you will find it in Part 11, Leaflet 11-22 "Experience from Incidents" Appendix 34-3 The link below should take you to the CAP
CAP 562: Civil Aircraft Airworthiness Information and Procedures (CAAIP) | Publications | CAA