In today's industrial Internet of Things systems, functional safety communication protocols are widely adopted to transmit safety protocol data unit (SPDU). While the guessing random additive noise decoding (GRAND) algorithm can improve the reliability of cyclic redundancy check (CRC)-coded SPDU, the decoding complexity of long SPDU remains too high for practical deployment. To address this, we extend the GRAND-based joint error detection and correction (JEDeC) strategy to long SPDU and propose a JEDeC-based packet fragmentation/assembling mechanism that fragments a long SPDU into multiple short SPDUs for parallel correction of erroneous bits. We clarify the decoder input settings and channel model used in this work. We show that the proposed approach achieves tractable decoding complexity and latency: for an assembled SPDU length of 1024 bits, fragment length of 64 bits, CRC signature length of 32 bits, and maximum error-correction capability of 4 bits, under a representative bit error rate (BER) Pe=10−3P_{e}=10^{-3}, the BER is reduced from 10−310^{-3} to 1.49×10−81.49\times 10^{-8}, the packet error rate from 6.46×10−16.46\times 10^{-1} to 8.11×10−78.11\times 10^{-7}, and the residual error probability from 6.36×10−116.36\times 10^{-11} to 3.42×10−163.42\times 10^{-16}, with an average of 2.86×1032.86\times 10^{3} guessing attempts per assembled SPDU. These results indicate that the fragmentation/assembling mechanism can substantially improve safety-communication dependability with implementable cost.